It never occured to me, so I can only ask questions… The list of alarms are saved in ~/.timed/events.data. There even is a backup of this file in this directory. I guess when the timed daemon is started, this file is read and alarm restored accordingly.
If it happens to you again, may you look at this file, check that it exists, is readable and contains accurate data (its a plain text file) ?
EDIT: oops, sorry @canne I didn’t read your summary yet when writing this…