I figured it out, but in case anyone else runs into this I wanted to post the solution.
I realized that I could leave print statements in my script and they write the output to the nas.log file so I can view the live data. In doing so, I printed the full alarm table and realized that all of the time-related objects are in epoch time, and time_origin is not one of them. Below is a full dump on a live alarm for example. I'm not sure how time_origin works in my test scripts that are run manually, but not in auto-operator triggered scripts, but I'll leave that for another day.
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: root:
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: robot:hub-01
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: source:10.20.30.40
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: origin:HUB-01
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: arrival:1439940148
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: hostname:HUB-01
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: level:3
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: aots:1439940573
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: profile:Set Custom3-5
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: visible:1
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: sid:1.1.15
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: severity:minor
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: domain:LAB-UIM
Aug 18 16:29:33:841 [6808] nas: Set Custom3-5: nimts:1439940147
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: subsys:Other
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: message:This is a test message
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: tz_offset:25200
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: nas:HUB-01
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: hub:HUB-01
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: suppcount:0
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: nimid:LD32845607-94999
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: event_type:2
Aug 18 16:29:33:842 [6808] nas: Set Custom3-5: rowid:1
So, I modified my logic to use the nimts element and converted it to local time.
local al = alarm.get()
local update = ()
update.nimid = al.nimid
update.custom_1 = os.date("%c", al.nimts)
alarm.set(update)