Add option to convert timestamp to local offset

I use the `--timestamp` option to display the log time a lot.

However, it always shows the log line's timestamp in UTC.

I would love the ability to convert the timestamp to my local offset.

It doesn't need to be a fully localised time. ISO-8601 is fine for me, just adjust the time and offset.

Start by tracing the --timestamp option; the issue does not identify a file or test. The change is complete when timestamp output can be adjusted to the user's local offset while retaining an ISO-8601-compatible format.

Written by the indexing model from the issue text.