Sort of a hacky hammer, but
* The original design of the analytics system mistakenly attempted to play
nicely with non-UTC datetimes.
* Timezone errors are really hard to find and debug, and don't jump out that
easily when reading code.
I don't know of any outstanding errors, but putting a few "assert this
timezone is in UTC" around will hopefully reduce the chance that there are
any current or future timezone errors.
Note that none of these functions are called outside of the analytics code
(and tests). This commit also doesn't change any current behavior, assuming
a database where all datetimes have been being stored in UTC.
interval refers to a time interval, and frequency refers to something that
semantically means something closer to 'hourly' or 'daily'.
Currently, interval can have values 'hour', 'day', or 'gauge', and frequency
can only have values 'hour' and 'day'.
