Check use of dates/times/timezones is correct

[neiljp]

This may be straightforward, or require a deep dive to understand, but it's important we get this right.

The two points which suggest we may need to address this so far are:

• Enabling the DTZ rules in ruff raises some warnings/errors
• The tzlocal package went through a breaking change from v2 to 3 to 4, and I've not checked if we use the subset of features which are safe or remained unchanged

Please discuss in a new topic with this PR reference as a suffix, if you plan to work on this.