To determine time on site simply enter =I2-H2 and format as needed

If you want to keep the cells empty as long as one of the cells is blank, try

=IF(or(I2="",H2=""),"",I2-H2)

To summarize at the end of the month, a Pivot Table would to the trick

PS if anybody stays more than 24 hrs on site format as [h]:mm:ss