Here are the comments in the code.
// We must use UNSPECIFIED here, or inserting a Date-with-timezone into a
// timestamptz field does an unexpected rotation by the server's TimeZone:
//
// We want to interpret 2005/01/01 with calendar +0100 as
// "local midnight in +0100", but if we go via date it interprets it
// as local midnight in the server's timezone:
// template1=# select '2005-01-01+0100'::timestamptz;
// timestamptz
// ------------------------
// 2005-01-01 02:00:00+03
// (1 row)
// template1=# select '2005-01-01+0100'::date::timestamptz;
// timestamptz
// ------------------------
// 2005-01-01 00:00:00+03
// (1 row)
Apparently while dates don't store timezone information we do accept timezone info in date literals ?
