Rat5
I am quoting the UK CAA definition
22 ABSOLUTE LIMITED ON FLYING HOURS
A person shall not act as a member of the flight crew of an aircraft if at the beginning of the flight the aggregate of all previous flight times:
(a) during the period of 28 consecutive days expiring at the end of the day on which the flight begins exceeds 100hrs; or
(b) during the period of 12 months, expiring at the end of the previous month exceeds 900hrs.
It follows therefore that you start a flight (not a series of flights) if you have exceeded 100hr within the last 28 days and the 900hrs is not a rolling year but a calendar 12 months.
Interestingly there is nothing to say that the 900hrs is wiped clean if you join a new employer so when you take up new employment you should really give your new employer a copy of your logbook to enable them to comply (if the 900hrs is critical).