Ah that makes sense! Mine were delivered in February and I couldn't find the date anywhere. Thanks!For me it was the same as the date of activity "Security Screening: Not started" in the "ACTIVITY" section.
The problem is that in the recent notes (starting 2018) they started redacting even more info, including those dates and all the security activities...
If your notes are like that, then you may try to look in the "NOTES" section at the very end and find the most recent note with the Label "Adm/<Redacted>" (or just "Adm"). It might be the security screening starting date (at least it matches in my notes).
But you can't really tell it for sure from 2018 GCMS notes, because they just don't seem to show it explicitly anymore... I can see it in my 2017 notes, but not in 2018.