Rather than storing the timestamp in the Guild Leader's public note, it could instead be stored somewhere in the guild info, such as on the last line, with a disclaimer to not modify that line.

This would make it less risky to allow non-officers to modify public notes. and open it up for other fun info.

I'm not sure how doable this is, but I do remember 'dkp (reloaded)' doing something similar for storing epgp values back in the day.

In 2.1 the timestamp has been removed entirely. Rather than storing it in the game at all it's stored as a "seed" in everyones DKP and Loot history tables. On login it's automatically propagated to the rest of the guild.

Fantastic!