I have a column for “Completed?” with a checkmark option. When this is checked, I’d like for that column (“When complete?”) to populate the time it was checked.
There could also be some overriding formats in the settings for the field, an example screenshot below. In other words, the formula could be setting the value as Central time, but the field is configured to show GMT.
I think you may be running into issues because you’re referencing your checkbox field, {Completed?} in your DATETIME_FORMAT() formula instead of the LAST_MODIFIED_TIME() for {Completed?} Written that way, the formula is trying to format a date/time value from what the checkbox returns (either a “0” or “1”), instead of a true date/time.
As long as the box isn’t checked for a record, there will be no date. Once it’s checked, it will receive the proper date, and it can be formatted as desired via the formula’s Formatting tab.