36 months is the qualifying period. Northern Ireland is obviously the one that immediately springs to mind as the main contributor. However in addition, any Op Tour that would have seen the award of GSM 1962 will contribute eg Op HAVEN (N Iraq), Armilla Patrol (Persian Gulf) etc. Any other tour where a different medal is authorised eg Bosnia/ Cyprus/ Kosovo NATO or UN medal does not qualify. I believe that the new Op service medal is also a non-qualifier.
Nearly right OPFOR but the qualifying period is 1080 days - the equivalent of 36 x 30 day months but actually 2 yrs 11 months and approx 2 weeks. I know that sounds petty but I know people who have failed to get it for the sake of a few days.
I know someone may have said this before but to my understanding any tour which qualifies for a "British Medal" counts towards days for the ACSM. At the moment that includes NI, Sierra Leone, Afghanistan and Iraq. The days can be built up over as long as it takes and once you reach the 1080 days you can apply to be awarded it. I am also sure that a tour in the Falklands counted until recently.