[Madmaxneo]

Don't get me wrong, I don't sweat the small stuff. I have never actually kept track of things like in game time.
But over the years and the many campaigns I have run the campaign cohesion can suffer drastically.
Right now it isn't a serious problem but over time as things happen it can be. You're right that it is primarily for me, the GM. But it also eventually falls into the players hands in one form or another though they may never know it while suffering the effects of it.
I like to run long running campaigns. I want to get into a campaign that will run for years to come. A calendar feature will definitely improve the feel and structure of the campaign. It will also help when running adventures that happen in the past.