When creating a custom datetime field the field type is actually 'datetimecombo' and not 'datetime'
I think this has something to do with the field appearing in advanced search. Side note: what a worthless search feature as you have to specify an exact date and time to search for as far as I can tell. How stupid.
Anyway, what happens is in a workflow if you have an action to copy a value from a datetime field to a custom field, it won't work because the custom field type is datetimecombo. There's no way in studio to override this.
What's the most elegant way around this issue?