I'm guessing you are using the 'Add an event report'? It always shows the calendar on the current year, month and date. That calendar is the 'Start Date'. You have to use the back arrows to get to the year and month you need and then click on a date that applies. There is an optional 'End date' where you type in your date but if you didn't change the start date, it defaults to the date you added it.
