Normally, it is the date of your last promotion. But for new accessions, it depends on how you were commissioned. Unless things have changed since I was commissioned a hundred or so years ago, OTS and USAFA guys DOR is they day they are commissioned, as that is their first day of active duty as an officer. However, AFROTC commissionees are delayed coming on AD, so the DOR was split between the day they were commissioned and their first day of AD. That may have changed, but unless you are in a POW camp, what does DOR really matter anyway, especially when you are a Lt?
Cheers! M2