I've had a few questions lately regarding which special pays are paid by the month, and which ones are pro-rated by the number of days that the pay applies. I've tried to put together a list - please let me know if you have any additions or corrections for me:
Family Separation Allowance: pro-rated by days
Hardship Duty Pay: whole month
Imminent Danger Pay: whole month effective 1 February 2012, prorated by days, unless subject to actual hostile fire and other specific circumstances. See Imminent Danger Pay Changes for more information.
Sea Pay: pro-rated by the day
Tax-exemption for combat zone: whole month
Let me know if there are other questions, and I'll figure it out.