Why does Beeminder think it's already tomorrow?
On your Beeminder dashboard, each goal has a dropdown you can use to enter data for today or for yesterday. Sometimes Beeminder's idea of what "today" is does not match what day it actually is in your part of the world!
The most common cause is that the goal has its deadline set to earlier than midnight, and you're looking at the goal after that time has passed. The deadline time sets the boundaries of the Beeminder day, so that it doesn't necessarily line up with the calendar day. (See more examples on the deadline info page!) If your goals have varying deadlines, "today" might mean the 5th for some, but still the 4th for others. Make sure that you pick the right day the work should count for before you submit your data!