Flight Duty Regualtion Interpretation

[flynavy1]

Hi

My employer regularly schedules me for 60 flight hours in 15 days. According to governmental regulations, 60 hours is only allowed in any "half calendar monthly period".

My question to anyone is: what legally defines a half calendar monthly period:
any 15 day period (as I believe), or only the two periods of a month from 01st-15th and from 16th-31st (as my employer believes)?

Thanks for any thoughts.