Row No. Process UserName    ActivityName    Units   StartTime
1   XYZ UserA   Activity1   2   8/10/2017 11:40:37
2   XYZ UserA   Activity1   2   8/10/2017 11:41:39
3   XYZ UserA   Activity1   2   8/10/2017 11:42:41
4   XYZ UserA   Activity2   3   8/10/2017 11:43:44
5   XYZ UserA   Activity2   3   8/10/2017 11:49:37
6   XYZ UserA   Activity1   2   8/10/2017 11:54:21
7   XYZ UserA   Activity1   2   8/10/2017 11:58:21
8   XYZ UserA   Activity2   4   8/10/2017 11:59:21

Process UserName    ActivityName    Units   Time in Secs    Time per Unit
XYZ UserA   Activity1   4   487 121.75
XYZ UserA   Activity2   7   638 91.14

df$EndTime= c(df$StartTime[-1],0)
DT = df

CalculatedSummary = DT[,.(sum(TimeinSecs),.N),by=.(Process,UserName,ActivityName,Unit1)]