Add a timezone-specific variant of date_trunc().
date_trunc(field, timestamptz, zone_name) performs truncation using
the named time zone as reference, rather than working in the session
time zone as is the default behavior. It's equivalent to
date_trunc(field, timestamptz at time zone zone_name) at time zone zone_name
but it's faster, easier to type, and arguably easier to understand.
Vik Fearing and Tom Lane
Discussion: https://postgr.es/m/
6249ffc4-2b22-4c1b-4e7d-
7af84fedd7c6@2ndquadrant.com