Function next_month 

pub fn next_month(ms: i64) -> i64

The same day-of-month one month later, 00:00 UTC, in epoch ms; the year wraps. The day is clamped to 28 so an anniversary on the 29th–31st cannot skip a month into the wrong one — the same clamp month_start applies, so a period start it produced always has a matching end.