Here is a simple code that should work, you will have to define the values for hrs/min/sec and hrs2/min2/sec2 based upon the start and end time, but this should give you the concept:

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Start_Time = hrs*3600+min*60+sec

End_time =  hrs2*3600+min2*60+sec2 + (end_date - start_date) *24*3600

time_dif = end_time - Start_time

Total_Time_days = INT((time_dif)/(24*3600))
Total_Time_Hours =INT(MOD((time_dif),(24*3600))/3600)
Total_Time_Minutes=INT(MOD(MOD((time_dif),(24*3600)),3600)/60)
Total_Time_Secondds = MOD(MOD(MOD((time_dif),(24*3600)),3600),60)

Total_time = "%Total_Time_days% Days %Total_Time_Hours%:%Total_Time_Minutes%:%Total_Time_Secondds%"