Function month_start_before 

pub fn month_start_before(reset_ms: i64, offset_secs: i64) -> i64

The subscription month’s start from the meter’s own reset instant.

ARK’s monthly reset is the last second of the anniversary day (2026-11-19T23:59:59+08:00), so the cycle starts at 00:00 on that day-of-month in the previous month, in the reset’s own offset — the server speaks +08:00, and bucketing its instant as UTC would put the boundary eight hours off.