In other words, if a character hits level 5 on May 1st, for example, when that player does decide to use a +1, whether the day after that or six months later, it could be taken only from the sources that were available on the date the character reached level 5.
I can understand the spirit of this interpretation, and it's even workable, given that every character should have dates on their character logs that identify the specific dates that they received the XP from an adventure (or other source, like a claimed DM Quest) so that a player should be able to identify, if asked, the date the character gained the XP to reach any given level.
Though, technically, the date we should be using is the date that the character played its first adventure after reaching 5th level (or higher), since that is the 'official' determination of a character 'reaching 5th level' according to the FAQ:
FAQ v3.0 said:You are officially 5th level upon playing your first adventure at that level. Once the game begins you can’t rebuild your character, qualify for Faction Charity, etc.
So in effect, adopting this interpretation would mean that a character built during Season One (when there were no other rules options available), who reached 5th level and then played an adventure before the EEPC was released in March of 2015 would never** be able to select a '+1' rules source, since none existed before the character reached 5th level, and without the benefit of a rebuild, cannot be rebuilt to incorporate one.
** - Given that a 'free rebuild' was allowed at the time, such a character may have been rebuilt to make use of EEPC options -- such a character would be considered to have selected the EEPC as its '+1' rules source.
--
Pauper