Allowances reset at the start of each calendar month, measured in UTC. That is the same moment for everyone regardless of where you are, which avoids the situation where two people on the same plan get different reset times.
In practice, if you are in India the reset lands at 5:30am local; on the US west coast it is 5pm the previous day. If you are planning a large piece of work around a reset, check what UTC midnight is where you are.
Unused words do not carry over. An allowance is per period rather than a balance you accumulate, so a quiet month does not bank capacity for a busy one.
That model suits steady work better than bursts. If your writing arrives in concentrated periods — a dissertation month followed by nothing — you may find you need a higher tier than your average monthly volume suggests, because the average is not what you are buying against.
The reset is based on the calendar month rather than your signup date, so everyone's allowance refreshes on the same day.
If you run out mid-month, the options are waiting for the reset or moving up a tier. Changing plan takes effect immediately rather than at the next reset.
Detection and humanizer allowances reset together on the same schedule, so there is no need to track two dates. Nothing partially rolls over: both counters return to their full plan value at the same moment.