i want to get sum of time as hh:mi:ss

Hi,

My client is  working on time & attendance software and it required me to get the sum of overtimes,late coming and early going.actually i have the 3 character fields for overtime, late, early and at the end of the month i want to get the summary of  these fields in format of hh:mi:ss.For ex.  in overtime i may have different values for different days in format of hh:mi:ss and i want to calculate the summary of overtime for complete month.  and i am using sum function but its giving me the error as invalid number.so please if could you please resolve