Database design for a time tracking application
Have a look at the Timesheet model on databaseanswers.org. This covers it nicely

Simply, a table with


Date
User (FK to User table)
Project (FK to Project table)
Time worked


The first 3 columns are PK.

Now, do you want to:


log activity eg Analysis, Coding, testing separately?
log project phases separately? 
log non-project work eg training, holidays (eg user focus or project focus?)
...