I would use a join table that connects user to a project and also stores the additional data

      user   |    project    |    time     |   date
    -------------------------------------------------
        1            39           360        27/09/2011
        2            4            60         27/09/2011
        6            34           500        27/09/2011
        5            34           320        27/09/2011
        12           37           720        27/09/2011
        7            34           50         27/09/2011

The time is here in minutes, but may also be in seconds (although that might be over-engineered).

Of course, your keys for user and project might be different, but I think you get what I mean.