I have users input number of hours for a particular month/day
What do they input the numbers into? A cell, a textbox? or . . .
What version of excel are you using?

and it updates a table in another sheet

What updates the table?
How does it update the table?

However, when they go to enter hrs for january 2, january 1 info gets set back to zero,

How does it get set back to zero?