How to Validate Date and Time Strings

To validate date and time formats, we can define a helper function that uses strftime codes to check for matching strings.

In the following example, we use the code %Y-%m-%d to check for dates that match the pattern YYYY-MM-DD:

from datetime import datetime
from datatest import validate


def strftime_format(format):
    def func(value):
        try:
            datetime.strptime(value, format)
        except ValueError:
            return False
        return True
    func.__doc__ = f'should use date format {format}'
    return func


data = ['2020-02-29', '03-17-2021', '2021-02-29', '2021-04-01']
validate(data, strftime_format('%Y-%m-%d'))

Date strings that don’t match the required format are flagged as Invalid:

Traceback (most recent call last):
  File "example.py", line 17, in <module>
    validate(data, strftime_format('%Y-%m-%d'))
datatest.ValidationError: should use date format %Y-%m-%d (2 differences): [
    Invalid('03-17-2021'),
    Invalid('2021-02-29'),
]

Above, the date 03-17-2021 is invalid because it’s not well-formed and 2021-02-29 is invalid because 2021 is not a leap-year so the last day of February is the 28th—there is no February 29th in that calendar year.

Strftime Codes for Common Formats

You can use the following format codes with the function defined earlier to validate many common date and time formats (e.g., strftime_format('%d %B %Y')):

format codes	description	example
%Y-%m-%d	YYYY-MM-DD	2021-03-17
%m/%d/%Y	MM/DD/YYYY	3/17/2021
%d/%m/%Y	DD/MM/YYYY	17/03/2021
%d.%m.%Y	DD.MM.YYYY	17.03.2021
%d %B %Y	DD Month YYYY	17 March 2021
%b %d, %Y	Mnth DD, YYYY	Mar 17, 2021
%a %b %d %H:%M:%S %Y	WkDay Mnth DD H:M:S YYYY	Wed Mar 17 19:42:50 2021
%I:%M %p	12-hour time	7:42 PM 1
%H:%M:%S	24-hour time with seconds	19:42:50

In Python’s datetime module, see strftime() and strptime() Format Codes for all supported codes.

Footnotes

1

The code %p expects the system locale’s equivalent of AM or PM. For example, the locale en_US uses “AM” and “PM” while the locale de_DE uses “am” and “pm”.