At my airline, your longevity date and date of hire are not the same. They go by the day you start training. I wonder if the military prevented you from beginning training if they would still honor the day you were supposed to begin as your longevity date.
There might be a slight confusion of terms here.
Most places, "Date of Hire" refers to the first day you report to work, ie, report to training. Therefore it's the same as your longevity date.
The day you got the phone call, letter, etc.. doesn't really factor in.
As far as retaining position, pay, and seniority, etc... I believe there are laws protecting servicemembers if they are activated or deployed.
If you're thinking about getting in, ask your recruiter. Odds are they have the scoop on the resources so you can check this out yourself. If you're already in, contact your JAG office. They'll know.