Send Email Scheduling not working

Hello Andy,
I just tried to schedule an email.
I unticked the “Send Now” box and clicked the calendar icon, expecting there to be a dropdown date selector, but none appeared.
I typed tomorrow’s date in the box (UK format, 30/04/2024) and hit send.
The email arrived immediately.
I don’t think this will make any difference to the solution, but this was on my site using “Other SMTP” not Mailersend.

If, in fixing, it would be straightforward to add a send TIME as well as send DATE, that would be fabulous!
Thanks very much,