how to get the time diffrence from givn times in file

I have file with below data, I want to get the highest time difference from this using PowerShell.

STARTTIME:2018-12-01 04:13:15
ENDTIME:2018-12-01 04:17:15
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STARTTIME:2018-12-01 04:11:15
ENDTIME:2018-12-01 04:13:15
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STARTTIME:2018-12-01 04:10:15
ENDTIME:2018-12-01 04:10:40