Time Tracker - I need a macro that calculates the time spent on an activity

Hi,

Can anyone help me with a macro that calculates the time spent on an activity? I have attached a spread sheet. If i choose training, then the start time should automatically populate. Then when i choose the second activity, it means that my training is over and i have moved on to the next activity. So it should populate the end time for training and the start time for the second activity (meeting in the spread sheet). Please help