Scheduled email handler ISO date/time field doesn't accept ISO8601-formatted date and time strings

After diving into the code a bit more I've realized, rather than broadening out to accepting just ISO 8601 and MYSQL DATETIME formats, it probably makes more sense to accept anything PHP's strtotime() function can consume. The downstream code just throws this value against strtotime(), which accepts many formats, including both MYSQL DATETIME and ISO 8601. As such, limiting entry to just MYSQL DATETIME and/or ISO 8601 seems unwarranted and adds what feels like unnecessary complexity to the validation method.

The approach I'm taking with the MR currently is to drop the "ISO date" stuff and instead ask for a date or date-time string in a format compatible with PHP's strtotime() function, keeping the current MYSQL DATETIME YYYY-MM-DD or YYYY-MM-DD HH:MM:SS as the suggested format. This seems to fix the bug in the least disruptive way, bringing in actual ISO 8601 date format support, keeping MYSQL DATETIME support, slightly reducing validation complexity, and adding support for many other date formats.

I'll post the MR for review after I've confirmed tests pass locally.

MR 425 opened. Status changed to needs-review. Proposed resolution and UI changes sections updated to match the resolution proposed in MR 425. Immutable diff (patch) attached here capturing the current state of this MR.

Merged latest 6.2.x changes into MR branch. Removed unused use statement. Added note about the default timezone that will be used to interpret the custom send-on date if not provided in the date string (so a source-code-dive isn't necessary to figure that out). Current state of the MR is attached, along with a screenshot of the changed field.

Automatically closed - issue fixed for 2 weeks with no activity.