Here are some possible causes:

A timezone issue

If your dates are off by only a day or a few hours, it could be a time zone issue.

You may need to use the DATETIME_PARSE() function on your date and set the locale.

DATETIME_PARSE("2/23/2020", "MM/DD/YYYY", 'locale')

Here is a modified version of your snippet before URL encoding. Be sure to change 'locale' to the actual locale you want.

IS_AFTER({Last Modified Date}, DATETIME_PARSE( "2/23/2020", "MM/DD/YYYY", 'locale'))

You also should not need to compare the result of IS_AFTER to 0. The function returns 0 (false) or 1 (true).

Calculated fields

The {Last modified time} field type is based on only changes made by a user in a non-computed field. If a value changes in a computed field (such as in a lookup or rollup), the {Last modified time} will not change, even though the record will appear to change in the view.

Different field

The {Last modified time} field might be based off a different field than you expect.

DATETIME_PARSE("2/23/2020", "MM/DD/YYYY", 'locale')

IS_AFTER({Last Modified Date}, DATETIME_PARSE( "2/23/2020", "MM/DD/YYYY", 'locale'))

Thanks!

I realised on reading your response that I’m comparing IS_AFTER result as >=0 :woman_facepalming:

Changed it now.