Review and add doc around timezone handling

It sounds like the docs aren't clear around us returning UTC datetimes rather than localtime (see issue #35) so we should review and make it less confusing.

Need to include the reasoning on why we return UTC and not localtimes (see SO question linked in #35 and use that summary)

Start by reading issue #35 and the linked Stack Overflow question, then locate the project documentation covering timezone handling. Update that documentation to explain that returned datetimes use UTC rather than local time and include the referenced reasoning; done means the distinction and rationale are clear to readers.

Written by the indexing model from the issue text.