The farmers did not care. Nile floods at sometime, then it goes back then you do all the normal things until you harvest. Repeat. So they really did not need calendar for that.

And on other hand the clerks had enough time to make writing system really hard to learn so such thing would make things even better for them...

its always hot there, so no wonder they didn't notice winter shift (joking).

This is actually partially part of it - if you're tracking your activities based on natural phenomena, you don't need to worry about a calendar - just things like "days since full moon" or "weeks since the flood".