Concalendrical reference point

Note that this thread is of marginal topicality, but I can't think of
a better place to ask.

I've been trying to develop a calendar.  It's just a standard calendar
for use on Earth, with no speacal concultural associations.  I'm
fairly happy with the mechanics of the thing, but one question
troubles me - from what date to start the long count of years?  I
could just start it from when I finish the calendar (or that year,
anyway - I'm thinking of having the year start at the vernal equinox,
like the Vorlin calendar) but that seems to perhaps attach too much
importance to the creation of the calendar itself.  I'm unable to
decide on any one event of such importance in history.  So one idea I
had was to simply take the earliest recorded event which can be
precisely dated (at least to the year) with a reasonable degree of
certainty. So my question is, does anyone know what that event is?
(Preferably not an astronomical observation, as we only know these
with accuracy because we can project them back in time, and could do
this in theory regardless of contemporary records - but another event
recorded with reference to an astronomical event would be fine.)

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