Previous to December 31, 2011, Marines deployed to hostile zones were given $250 extra a month. Even if you had only been in country for part of the month, you would be paid the full 250. For example: If the Marine landed in Country on the 21st of the month, they would immediately receive the full imminent danger pay amount. Same if a Marine leaves the country on the 2nd of the month; they would still receive the full 250 dollar benefit.
However, with new legislation, as of February 1st, imminent danger pay will be paid on a per diem basis of $7.50 per day in a hostile fire zone.
Please be aware that if you or your Marine were paid the previous full rate despite only being in country for only part of the month between the dates of December 31, 2011 and February 1, 2012, that extra pay will be taken out of your paychecks. According to this article from the Marines, this can happen as early as the February 15th paycheck.
HOWEVER, the branches across the board are working to allow the service members paid between this time to keep the money. There is nothing that any of you need to do to get this waived. If it happens, it will happen automatically.
But in case it does, be prepared. The easiest way to figure out how much will be taken out of your paychecks is to count how many days the service member wasn't in country and multiply that by 7.50. Example: Service member leaves Afghanistan on January 14. There are 30 days in January. They were officially outside of imminent danger for 16 days. 16 x 7.50= 120.
If you anticipate any of this causing financial hardship, please see the Navy/Marine Corps relief society to get some sort of assistance.