Check startDate and endDate are in local time

[nickevansuk]

Due to the potential confusion of events not being outputted in local time, a check should be made to ensure they are actually local at source

This library can be used to facilitate this: https://github.com/evansiroky/node-geo-tz

This is particularly important for booking to ensure that times presented are always accurate.

Ref: openactive/realtime-paged-data-exchange#88, openactive/modelling-opportunity-data#197