In my case, SSDI used the date my spouse died when I was sent out from IRAQ on Emergency Leave to attend to this sudden and unexpected event.
I have read other posts where the date used by SSDI was the day the SM got transferred to a WTU or the date the SM was MedEvac'ed from the theater.
What SSDI is looking for is the date you were no longer performing the duties of your MOS. Being transferred to a WTU and the date of transfer on your order is a good one if you don't have any other specific date prior to that where you were not performing your MOS.
You can file for SSDI and eventually get approved for SSDI while on AD orders. Just be sure to go to a CPA or tax person BEFORE you spend that SSDI retro check money as it will be taxed if you have "working wages" over 1,000 per month or so. And all AD wages do count as taxable income earned - even if in a WTU.
nwlivewire