Case Study: ISO 8601 Date + Time

Case Study: ISO 8601 Date + Time#

Let us make use of what we have explored so far in a more elaborate case study. ISO 8601 is an international standard for exchange and communication of date and time-related data, providing an unambiguous method of representing calendar dates and times.

In this chapter, we will define a full Fandango spec for ISO 8601 date and time formats, ensuring both syntactic and semantic validity. To make matters more interesting, we will create the spec programmatically - that is, by having a number of Python functions that generate the spec for us.

Creating Grammars Programmatically#

We start with a number of functions that help us create fragments of the grammar.

make_rule() creates a grammar rule from symbol and its possible expansions:

import sys

def print_s(s: str) -> str:
    print(s, end="")
    return s

def make_rule(symbol: str, expansions: list[str], sep: str = '') -> str:
    return print_s(f"{sep}<{symbol}> ::= " + " | ".join(expansions) + "\n")

make_rule("start", ["<iso8601datetime>"]);  # a final ";" suppresses result

<start> ::= <iso8601datetime>

We can also use make_rule() to quickly create a list of expansions:

make_rule("digit", [f"'{digit}'" for digit in range(0, 10)]);

<digit> ::= '0' | '1' | '2' | '3' | '4' | '5' | '6' | '7' | '8' | '9'

These additional functions help in adding non-grammar elements such as headers, code, and constraints:

def make_header(title: str) -> str:
    return print_s(f"\n# {title}\n")

def make_comment(comment: str, sep: str = '') -> str:
    return print_s(f"{sep}# {comment}\n")

def make_constraint(constraint: str) -> str:
    return print_s(f"where {constraint}\n")

def make_code(code: str) -> str:
    return print_s(f"{code}\n")

make_header("ISO 8601 grammar");

# ISO 8601 grammar

Spec Header#

Let us create a Fandango spec. For now, we store the final spec in the iso8601lib string variable, to be saved in a file at the end. Every time we add a new fragment, the new fragment will be output here, so we can see the actual content of iso8601lib incrementally.

iso8601lib = make_header("ISO 8601 date and time")
iso8601lib += make_comment("Generated by docs/ISO8601.md. Do not edit.")

# ISO 8601 date and time
# Generated by docs/ISO8601.md. Do not edit.

We will need the datetime library below, so we import it.

iso8601lib += make_code("\nimport datetime\n")

import datetime

Date#

We start with a <start> symbol. An ISO 8601 date/time spec starts with a date, and an optional time, separated by T:

iso8601lib += make_rule("start", ["<iso8601datetime>"])

iso8601lib += make_rule("iso8601datetime",
                    ["<iso8601date> ('T' <iso8601time>)?"])

<start> ::= <iso8601datetime>
<iso8601datetime> ::= <iso8601date> ('T' <iso8601time>)?

A date can either be a calendar date, but also a week date or an ordinal date.

iso8601lib += make_rule("iso8601date",
                        ["<iso8601calendardate>",
                         "<iso8601weekdate>",
                         "<iso8601ordinaldate>"], sep='\n')

<iso8601date> ::= <iso8601calendardate> | <iso8601weekdate> | <iso8601ordinaldate>

Calendar Dates#

An ISO 8601 calendar date has the format YYYY-MM-DD, but can also be YYYY-MM or YYYYMMDD. The year can be prefixed with + or - to indicate AC/BC (or CE/BCE) designators.

iso8601lib += make_rule("iso8601calendardate",
                    ["<iso8601year> '-' <iso8601month> ('-' <iso8601day>)?",
                     "<iso8601year> <iso8601month> <iso8601day>"], sep='\n')
iso8601lib += make_rule("iso8601year", ["('+'|'-')? <digit>{4}"])

<iso8601calendardate> ::= <iso8601year> '-' <iso8601month> ('-' <iso8601day>)? | <iso8601year> <iso8601month> <iso8601day>
<iso8601year> ::= ('+'|'-')? <digit>{4}

Months#

Months are easy:

iso8601lib += make_rule("iso8601month",
                    [f"'{month:02d}'" for month in range(1, 13)])

<iso8601month> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12'

Days#

As with months, we define each day individually to avoid biasing the grammar. The fact that some months have only 28, 29, or 30 days will be handled later on in a constraint.

iso8601lib += make_rule("iso8601day",
                    [f"'{day:02d}'" for day in range(1, 32)])

<iso8601day> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31'

Tip

For testing purposes, it makes sense to test with extreme values - in our case, January 1, February 28 or 29, and December 31.

Week Dates#

ISO 8601 also allows specifying dates by week numbers (1 - 53) and optional days of the week (1 - 7). 2025-W12-1 is the Monday of the 12th week in 2025.

iso8601lib += make_rule("iso8601weekdate",
                    ["<iso8601year> '-'? 'W' <iso8601week> ('-' <iso8601weekday>)?"], sep='\n')
iso8601lib += make_rule("iso8601week",
                    [f"'{week:02d}'" for week in range(1, 54)])
iso8601lib += make_rule("iso8601weekday",
                    [f"'{weekday:1d}'" for weekday in range(1, 8)])

<iso8601weekdate> ::= <iso8601year> '-'? 'W' <iso8601week> ('-' <iso8601weekday>)?
<iso8601week> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53'
<iso8601weekday> ::= '1' | '2' | '3' | '4' | '5' | '6' | '7'

Ordinal Dates#

ISO 8601 also allows specifying dates by day numbers (1 - 366). 2000-366, for instance, specifies the last day in 2000 (which was a leap year). Again, we specify each day programmatically (and individually).

iso8601lib += make_rule("iso8601ordinaldate",
                    ["<iso8601year> ('-'? <iso8601ordinalday>)?"], sep='\n')
iso8601lib += make_rule("iso8601ordinalday",
                    [f"'{day:03d}'" for day in range(1, 367)])

<iso8601ordinaldate> ::= <iso8601year> ('-'? <iso8601ordinalday>)?
<iso8601ordinalday> ::= '001' | '002' | '003' | '004' | '005' | '006' | '007' | '008' | '009' | '010' | '011' | '012' | '013' | '014' | '015' | '016' | '017' | '018' | '019' | '020' | '021' | '022' | '023' | '024' | '025' | '026' | '027' | '028' | '029' | '030' | '031' | '032' | '033' | '034' | '035' | '036' | '037' | '038' | '039' | '040' | '041' | '042' | '043' | '044' | '045' | '046' | '047' | '048' | '049' | '050' | '051' | '052' | '053' | '054' | '055' | '056' | '057' | '058' | '059' | '060' | '061' | '062' | '063' | '064' | '065' | '066' | '067' | '068' | '069' | '070' | '071' | '072' | '073' | '074' | '075' | '076' | '077' | '078' | '079' | '080' | '081' | '082' | '083' | '084' | '085' | '086' | '087' | '088' | '089' | '090' | '091' | '092' | '093' | '094' | '095' | '096' | '097' | '098' | '099' | '100' | '101' | '102' | '103' | '104' | '105' | '106' | '107' | '108' | '109' | '110' | '111' | '112' | '113' | '114' | '115' | '116' | '117' | '118' | '119' | '120' | '121' | '122' | '123' | '124' | '125' | '126' | '127' | '128' | '129' | '130' | '131' | '132' | '133' | '134' | '135' | '136' | '137' | '138' | '139' | '140' | '141' | '142' | '143' | '144' | '145' | '146' | '147' | '148' | '149' | '150' | '151' | '152' | '153' | '154' | '155' | '156' | '157' | '158' | '159' | '160' | '161' | '162' | '163' | '164' | '165' | '166' | '167' | '168' | '169' | '170' | '171' | '172' | '173' | '174' | '175' | '176' | '177' | '178' | '179' | '180' | '181' | '182' | '183' | '184' | '185' | '186' | '187' | '188' | '189' | '190' | '191' | '192' | '193' | '194' | '195' | '196' | '197' | '198' | '199' | '200' | '201' | '202' | '203' | '204' | '205' | '206' | '207' | '208' | '209' | '210' | '211' | '212' | '213' | '214' | '215' | '216' | '217' | '218' | '219' | '220' | '221' | '222' | '223' | '224' | '225' | '226' | '227' | '228' | '229' | '230' | '231' | '232' | '233' | '234' | '235' | '236' | '237' | '238' | '239' | '240' | '241' | '242' | '243' | '244' | '245' | '246' | '247' | '248' | '249' | '250' | '251' | '252' | '253' | '254' | '255' | '256' | '257' | '258' | '259' | '260' | '261' | '262' | '263' | '264' | '265' | '266' | '267' | '268' | '269' | '270' | '271' | '272' | '273' | '274' | '275' | '276' | '277' | '278' | '279' | '280' | '281' | '282' | '283' | '284' | '285' | '286' | '287' | '288' | '289' | '290' | '291' | '292' | '293' | '294' | '295' | '296' | '297' | '298' | '299' | '300' | '301' | '302' | '303' | '304' | '305' | '306' | '307' | '308' | '309' | '310' | '311' | '312' | '313' | '314' | '315' | '316' | '317' | '318' | '319' | '320' | '321' | '322' | '323' | '324' | '325' | '326' | '327' | '328' | '329' | '330' | '331' | '332' | '333' | '334' | '335' | '336' | '337' | '338' | '339' | '340' | '341' | '342' | '343' | '344' | '345' | '346' | '347' | '348' | '349' | '350' | '351' | '352' | '353' | '354' | '355' | '356' | '357' | '358' | '359' | '360' | '361' | '362' | '363' | '364' | '365' | '366'

Tip

For testing purposes, it makes sense to test with extreme values - in our case, 1, 365 and 366.

Time#

Now for time specifications. In ISO 8601, time is specified as HH:MM:SS, where HH comes in 24-hour format. MM and SS are optional, as is the colon separator in an “unambiguous” context.

iso8601lib += make_rule("iso8601time",
                    ["'T'? <iso8601hour> (':'? <iso8601minute> (':'? <iso8601second> (('.' | ',') <iso8601fraction>)? )? )? <iso8601timezone>?"], sep='\n')

<iso8601time> ::= 'T'? <iso8601hour> (':'? <iso8601minute> (':'? <iso8601second> (('.' | ',') <iso8601fraction>)? )? )? <iso8601timezone>?

Hours#

Hours normally go from 00 to 23, but 24:00:00 is allowed to represent midnight at the end of a day.

iso8601lib += make_comment("24:00:00 is allowed to represent midnight at the end of a day", sep='\n')
iso8601lib += make_rule("iso8601hour",
                    [f"'{hour:02d}'" for hour in range(0, 25)])

# 24:00:00 is allowed to represent midnight at the end of a day
<iso8601hour> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24'

Minutes#

Minutes go from 00 to 59. Nothing special here.

iso8601lib += make_rule("iso8601minute",
                    [f"'{minute:02d}'" for minute in range(0, 60)])

<iso8601minute> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53' | '54' | '55' | '56' | '57' | '58' | '59'

Seconds#

Seconds normally go from 00 to 59, but 60 can be used to represent leap seconds.

iso8601lib += make_comment("xx:yy:60 is allowed to represent leap seconds", sep='\n')
iso8601lib += make_rule("iso8601second",
                    [f"'{second:02d}'" for second in range(0, 61)])

# xx:yy:60 is allowed to represent leap seconds
<iso8601second> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53' | '54' | '55' | '56' | '57' | '58' | '59' | '60'

Seconds can be followed by , or a . (see <iso8601time>, above) and a fraction - an arbitrary number of digits.

iso8601lib += make_rule("iso8601fraction", ["<digit>+"])

<iso8601fraction> ::= <digit>+

Tip

Testing with extreme values would mandate times such as 20:15:00,99999999999999999999 to test for possible buffer overflows.

Time Zones#

Any time can be followed by a time zone designator. This is either Z, indicating Coordinated Universal Time (UTC), or an offset designating the time zone.

iso8601lib += make_rule("iso8601timezone", ["'Z'",
                                            "'+' <iso8601hour> (':'? <iso8601minute>)?",
                                            "'-' <iso8601hour> (':'? <iso8601minute>)?"], sep='\n')

<iso8601timezone> ::= 'Z' | '+' <iso8601hour> (':'? <iso8601minute>)? | '-' <iso8601hour> (':'? <iso8601minute>)?

Ensuring Validity#

So far, our dates and times are syntactically valid, but not necessarily semantically. We can easily create invalid dates such as 2025-02-31 (February 31) or invalid times such as 24:10:00 (10 minutes past midnight - only 24:00:00 is allowed). We could now add a number of additional rules to check for all these properties, and/or extend the grammar accordingly. However, we can also simply be lazy and have the existing Python datetime module check all this for us:

import datetime

def is_valid_iso8601datetime(iso8601datetime: str) -> bool:
    """Return True iff `iso8601datetime` is valid."""
    try:
        datetime.datetime.fromisoformat(iso8601datetime)
        return True
    except ValueError:
        return False

is_valid_iso8601datetime("2025-01-01T14:00")

True

is_valid_iso8601datetime("2025-02-31")

False

is_valid_iso8601datetime("2000-02-29T00:00:00")

True

With this, we can now add a constraint that limits our generator to only valid dates and times:

iso8601lib += make_constraint("is_valid_iso8601datetime(str(<iso8601datetime>))")

where is_valid_iso8601datetime(str(<iso8601datetime>))

iso8601lib += '''
def is_valid_iso8601datetime(iso8601datetime: str) -> bool:
    """Return True iff `iso8601datetime` is valid."""
    try:
        datetime.datetime.fromisoformat(iso8601datetime)
        return True
    except ValueError:
        return False
'''

Even Better Validity#

The Python datetime module has a some limitations, which extend to our spec. For instance, datetime does not support 24:00:00 as a valid time:

is_valid_iso8601datetime("2024-12-31T24:00:00")

False

Hence, our iso8601.fan spec may miss out a number of ISO 8601 features.

The dateutil alternative provides an ISO 8601 parser without these deficiencies.

Let us redefine is_valid_iso8601datetime() to make use of the dateutil parser:

def is_valid_iso8601datetime(iso8601datetime: str) -> bool:
    """Return True iff `iso8601datetime` is valid."""
    try:
        dateutil.parser.isoparse(iso8601datetime)
        return True
    except ValueError:
        return False

Let us see if the above example now works:

is_valid_iso8601datetime("2024-12-31T24:00:00")

True

This now works! How about the earlier examples?

is_valid_iso8601datetime("2025-01-01T14:00")

True

is_valid_iso8601datetime("2025-02-31")

False

is_valid_iso8601datetime("2000-02-29T00:00:00")

True

These also work. Let us fix the spec to use dateutil instead:

iso8601lib = iso8601lib.replace('datetime.datetime.fromisoformat', 'dateutil.parser.isoparse')
iso8601lib = iso8601lib.replace('import datetime', 'import dateutil  # See https://dateutil.readthedocs.io');

Fuzzing Dates and Times#

Our ISO 8601 spec is now complete. Let us write it into a .fan file, so we can use it for fuzzing:

open('ISO8601.fan', 'w').write(iso8601lib);

Here comes iso8601.fan in all its glory:

# ISO 8601 date and time
# Generated by docs/ISO8601.md. Do not edit.

import dateutil  # See https://dateutil.readthedocs.io

<start> ::= <iso8601datetime>
<iso8601datetime> ::= <iso8601date> ('T' <iso8601time>)?

<iso8601date> ::= <iso8601calendardate> | <iso8601weekdate> | 
<iso8601ordinaldate>

<iso8601calendardate> ::= <iso8601year> '-' <iso8601month> ('-' <iso8601day>)? 
| <iso8601year> <iso8601month> <iso8601day>
<iso8601year> ::= ('+'|'-')? <digit>{4}
<iso8601month> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' 
| '10' | '11' | '12'
<iso8601day> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' | 
'10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | 
'21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31'

<iso8601weekdate> ::= <iso8601year> '-'? 'W' <iso8601week> ('-' 
<iso8601weekday>)?
<iso8601week> ::= '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' | '09' 
| '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | '20' | 
'21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | '30' | '31' | 
'32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | '41' | '42' | 
'43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | '52' | '53'
<iso8601weekday> ::= '1' | '2' | '3' | '4' | '5' | '6' | '7'

<iso8601ordinaldate> ::= <iso8601year> ('-'? <iso8601ordinalday>)?
<iso8601ordinalday> ::= '001' | '002' | '003' | '004' | '005' | '006' | '007' | 
'008' | '009' | '010' | '011' | '012' | '013' | '014' | '015' | '016' | '017' | 
'018' | '019' | '020' | '021' | '022' | '023' | '024' | '025' | '026' | '027' | 
'028' | '029' | '030' | '031' | '032' | '033' | '034' | '035' | '036' | '037' | 
'038' | '039' | '040' | '041' | '042' | '043' | '044' | '045' | '046' | '047' | 
'048' | '049' | '050' | '051' | '052' | '053' | '054' | '055' | '056' | '057' | 
'058' | '059' | '060' | '061' | '062' | '063' | '064' | '065' | '066' | '067' | 
'068' | '069' | '070' | '071' | '072' | '073' | '074' | '075' | '076' | '077' | 
'078' | '079' | '080' | '081' | '082' | '083' | '084' | '085' | '086' | '087' | 
'088' | '089' | '090' | '091' | '092' | '093' | '094' | '095' | '096' | '097' | 
'098' | '099' | '100' | '101' | '102' | '103' | '104' | '105' | '106' | '107' | 
'108' | '109' | '110' | '111' | '112' | '113' | '114' | '115' | '116' | '117' | 
'118' | '119' | '120' | '121' | '122' | '123' | '124' | '125' | '126' 

| '127' | 
'128' | '129' | '130' | '131' | '132' | '133' | '134' | '135' | '136' | '137' | 
'138' | '139' | '140' | '141' | '142' | '143' | '144' | '145' | '146' | '147' | 
'148' | '149' | '150' | '151' | '152' | '153' | '154' | '155' | '156' | '157' | 
'158' | '159' | '160' | '161' | '162' | '163' | '164' | '165' | '166' | '167' | 
'168' | '169' | '170' | '171' | '172' | '173' | '174' | '175' | '176' | '177' | 
'178' | '179' | '180' | '181' | '182' | '183' | '184' | '185' | '186' | '187' | 
'188' | '189' | '190' | '191' | '192' | '193' | '194' | '195' | '196' | '197' | 
'198' | '199' | '200' | '201' | '202' | '203' | '204' | '205' | '206' | '207' | 
'208' | '209' | '210' | '211' | '212' | '213' | '214' | '215' | '216' | '217' | 
'218' | '219' | '220' | '221' | '222' | '223' | '224' | '225' | '226' | '227' | 
'228' | '229' | '230' | '231' | '232' | '233' | '234' | '235' | '236' | '237' | 
'238' | '239' | '240' | '241' | '242' | '243' | '244' | '245' | '246' | '247' | 
'248' | '249' | '250' | '251' | '252' | '253' | '254' | '255' | '256' | '257' | 
'258' | '259' | '260' | '261' | '262' | '263' | '264' | '265' | '266' | '267' | 
'268' | '269' | '270' | '271' | '272' | '273' | '274' | '275' | '276' | '277' | 
'278' | '279' | '280' | '281' | '282' | '283' | '284' | '285' | '286' | '287' | 
'288' | '289' | '290' | '291' | '292' | '293' | '294' | '295' | '296' | '297' | 
'298' | '299' | '300' | '301' | '302' | '303' | '304' | '305' | '306' | '307' | 
'308' | '309' | '310' | '311' | '312' | '313' | '314' | '315' | '316' | '317' | 
'318' | '319' | '320' | '321' | '322' | '323' | '324' | '325' | '326' | '327' | 
'328' | '329' | '330' | '331' | '332' | '333' | '334' | '335' | '336' | '337' | 
'338' | '339' | '340' | '341' | '342' | '343' | '344' | '345' | '346' | '347' | 
'348' | '349' | '350' | '351' | '352' | '353' | '354' | '355' | '356' | '357' | 
'358' | '359' | '360' | '361' | '362' | '363' | '364' | '365' | '366'

<iso8601time> ::= 'T'? <iso8601hour> (':'? <iso8601minute> (':'? 
<iso8601second> (('.' | ',') <iso8601fraction>)? )? )? <iso8601timezone>?

# 24:00:00 is allowed to represent midnight at the end of a day
<iso8601hour> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | '08' 
| '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | '19' | 
'20' | '21' | '22' | '23' | '24'
<iso8601minute> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | 
'08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | 
'19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | 
'30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | 
'41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | 
'52' | '53' | '54' | '55' | '56' | '57' | '58' | '59'

# xx:yy:60 is allowed to represent leap seconds
<iso8601second> ::= '00' | '01' | '02' | '03' | '04' | '05' | '06' | '07' | 
'08' | '09' | '10' | '11' | '12' | '13' | '14' | '15' | '16' | '17' | '18' | 
'19' | '20' | '21' | '22' | '23' | '24' | '25' | '26' | '27' | '28' | '29' | 
'30' | '31' | '32' | '33' | '34' | '35' | '36' | '37' | '38' | '39' | '40' | 
'41' | '42' | '43' | '44' | '45' | '46' | '47' | '48' | '49' | '50' | '51' | 
'52' | '53' | '54' | '55' | '56' | '57' | '58' | '59' | '60'
<iso8601fraction> ::= <digit>+

<iso8601timezone> ::= 'Z' | '+' <iso8601hour> (':'? <iso8601minute>)? | '-' 
<iso8601hour> (':'? <iso8601minute>)?
where is_valid_iso8601datetime(str(<iso8601datetime>))

def is_valid_iso8601datetime(iso8601datetime: str) -> bool:
    """Return True iff `iso8601datetime` is valid."""
    try:
        dateutil.parser.isoparse(iso8601datetime)
        return True
    except ValueError:
        return False

With this, we can now create a bunch of random date/time elements:

$ fandango fuzz -f iso8601.fan -n 10

6899-03-28
40570303
7306-11-14
1405-083T17-08
7550-W49-5T14:27
6964358
5201W27
4190
3661-01
3165-04-28

And we can use additional constraints to further narrow down date intervals:

$ fandango fuzz -f ISO8601.fan -n 10 -c 'int(<iso8601year>) > 1950 and int(<iso8601year>) < 2000'

1969-09-12
1953-04-18
1998-W31-5
1967-W24
1953-04-30
1970-12-22
1964-187
1988-W35-1
1989186T22:47:54,42712214087357775249526290419518242533119305513952671-23:32
1976-12-22

Or produce individual elements, again with individual constraints:

$ fandango fuzz -f ISO8601.fan -n 10 --start-symbol='<iso8601time>' -c '<iso8601hour> == "00"'

fandango:WARNING: Symbol <start> defined, but not used

00Z
T00
T004114
T00:44
0010
00:49:56,68-00
T00Z
0000-00
00:37:10Z
00

Try out more constraints for yourself! The generated ISO8601.fan file is available for download.

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