GROUPBYBIN

The GROUPBYBIN argument value can also accept date values though not officially supported and not all date/time parse patterns are available to describe the bin size. Date values are defined by millisecond so that 3,600,000 ms is equal to 1 hour. Simple time/date parse patterns that can be described in milliseconds return correct bin sizes.

Examples:
s = second, m = minute, h = hour, d = day

It isn't possible to create a "month" bin size but is possible to create a bin with a 30 day size.