The spec says:
> If the timestamp does not explicitly include any timezone information,
> it must be assumed to be in UTC.
> 2020-12-13T07:45:23 → 2020-12-13T07:45:23Z
This commit addresses the UTC fallback. Previously, when no timezone was
present, the Zulu timezone was not used when calculating the twt hash,
producing a different twt hash.