I've done so for this post. I wanted it to appear on 30th August (though in the UK as I type this it is after midnight so initially the date came up as the 31st, correctly of course. I went into the calendar icon and changed it back - a little bit of time travel and I didn't even leave my desk!).
But it led me to think about how calendars are organised. Just what system does your fictional world use for days, months, years etc? Does your fictional world have a time system we would recognise and, if not, on what do they base theirs? We follow a 24 hour day, which makes sense given we are following the pattern of night and day but of course length of days vary depending on what planet you're on. Where does your fictional world get its major light from, assuming it bases its time system on that?
In the past, calendars have been changed. In the UK we once lost 11 days because of an adjustment of this kind (and people protested about it too!). Who ultimately decides the system here and, if that is not part of your story, how does the way time is managed affect your characters as it is bound to do so? Any creature will get tired so there should be a pattern of activiity and rest, usually based on night and day patterns.