Date and Time Patterns

ISO 8601 — the canonical format

Always prefer YYYY-MM-DD and ISO 8601 timestamps when you control the format. The regex is trivial:

Date: \d{4}-\d{2}-\d{2}
Time: \d{2}:\d{2}:\d{2}
Datetime: \d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d+)?(?:Z|[+-]\d{2}:?\d{2})?

Common ambiguous formats

05/04/2026: is this May 4 (US) or April 5 (most of the world)? Regex can't tell. Don't accept ambiguous formats. Force ISO 8601, or accept multiple unambiguous formats explicitly.

If you must parse various formats, write multiple patterns and try each in order:

1. ISO: \d{4}-\d{2}-\d{2}
2. US slash: \d{1,2}/\d{1,2}/\d{4}
3. EU dot: \d{1,2}\.\d{1,2}\.\d{4}
4. Long: (Jan|Feb|...)\s+\d{1,2},?\s+\d{4}

Validation vs parsing

Regex tells you the SHAPE matches. It doesn't tell you the date is REAL. 2026-02-30 matches the pattern but Feb 30 doesn't exist. Real validation: datetime.strptime(s, fmt) in Python, new Date(s) in JS (then check !isNaN), time.Parse in Go.

Extracting dates from prose

For "find dates in this paragraph," use anchored alternation: \b(\d{4}-\d{2}-\d{2}|\d{1,2}/\d{1,2}/\d{4})\b. Be conservative — false positives in date extraction are common.

Code

Date patterns·python

import re
from datetime import datetime

# ISO date validation
DATE_ISO = re.compile(r'^\d{4}-\d{2}-\d{2}$')
bool(DATE_ISO.fullmatch('2026-05-04'))   # True
bool(DATE_ISO.fullmatch('2026/05/04'))   # False

# Validate AND parse
def parse_iso_date(s):
    if not DATE_ISO.fullmatch(s):
        return None
    try:
        return datetime.strptime(s, '%Y-%m-%d').date()
    except ValueError:
        return None  # caught Feb 30 and similar

parse_iso_date('2026-05-04')  # date(2026, 5, 4)
parse_iso_date('2026-02-30')  # None — shape OK, value invalid

# Full ISO datetime with timezone
DT_ISO = re.compile(
    r'^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(?:\.\d+)?(?:Z|[+-]\d{2}:?\d{2})?$'
)
bool(DT_ISO.fullmatch('2026-05-04T14:32:11Z'))            # True
bool(DT_ISO.fullmatch('2026-05-04T14:32:11.123+09:00'))   # True

# Find dates in prose
DATE_FIND = re.compile(r'\b\d{4}-\d{2}-\d{2}\b')
re.findall(DATE_FIND, 'logged on 2026-05-04 and 2026-05-05')
# ['2026-05-04', '2026-05-05']

ISO 8601 — the canonical format

Common ambiguous formats

Validation vs parsing

Extracting dates from prose

Code

External links

Exercise

Progress

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