This little bug has been bugging me for months now. But today I decided to fix this thing and here's what happens:

If you're using the desktop client of Windows Live Mail & try to add an event in the calendar when you "save" the event after entering all the details, the event won't be added in the calendar. The only way to add an event in the calendar in this scenario is to go to the online version, add the event & wait for the calendar to sync.

Here is the simplest of solution to this really annoying issue:

On the desktop -> Control Panel -> Clock, Language & Region -> Change the Date, Time, or number format -> set the year format for Short date to yyyy

To customize it use Additional settings. But essentially the short date should have a 4 character year.

Now when you try to add events in the calendar in WLM you should face no issues.