No comment so far on the use of the term "calendar year" in the annual limit. CAP 371 specifies no more than 900 hours in any 12-month period. So, why introduce the calendar year in its place?
Extreme case. Imagine the FTL calendar year starts at Jan 1. You start flying 100 hours a month on April 1 and keep that up and have 900 hours at next Jan 1. Then, the clock is reset, and you start again and if you still did 100 hours a month you would have, by next April, clocked up 1200 hours. Improbable, perhaps, why allow it if not to permit occasions when airlines want to get around the intent of the FTL? "Any 12-month period" is the only logical approach to FTL.