After a meeting is created you can always change the meeting time.

Go to the meeting screen for the subject meeting.

How do I edit a meeting?

On the left side of the screen, find the Meeting Data block.

Find the meeting date. If your access privileges allow you to change the time for the meeting, a Change Meeting Time link appears below the time.

Click the link to display a timepicker

Choose the new time and click the Change Meeting Time button.