Inconsistent date/time handling in CLIs

[tjtg]

As encountered in #1705 (comment), there are a variety of ways of handling CLIs with date/time inputs.

• Most are timezone naive, a few enforce use of the UTC "Z" timezone
• Some CLIs pass these onto internal processing functions as strings, others parse the datetime early and pass a python datetime.datetime object
• Some CLIs use lists of datetimes and this usage will need to be catered for

It would be desirable for the CLIs to use a single consistent approach to dealing with date/time inputs, rather than the current variety of approaches. See summary written by @benowen-bom in #1705 (comment) for further details of CLIs with date/time inputs.

Acceptance criteria:

• Agreed approach for handling date/time inputs to CLIs
• Code changes so that CLIs which are passed date/time strings as input use the agreed approach