2018-01-30 10:21:41
2018-01-17 12:22:50
2018-06-27 23:09:20
INVALID
INVALID
... for 800 Rows

2018-01-30 10:23:54
2018-01-17 13:23:45
2018-06-28 06:10:56
INVALID
INVALID
... for 800 rows

00:02:13
01:00:55
07:01:36
INVALID     //Where any instance of invalid in either file remain in the new file.
INVALID

string1="10:21:41"
string2="10:23:54"
StartDate=$(date -u -d "$string1" +"%s")
FinalDate=$(date -u -d "$string2" +"%s")
date -u -d "0 $FinalDate sec - $StartDate sec" +"%H:%M:%S"

> 00:02:13