You need to determine whether timestamp is in epoch format or string format. If they are string time you need to convert to epoch first. Try the following:
<Your Base Search>
| eval Time1=if(isnull(Time1),0,Time1)
| eval Time2=if(isnull(Time2),0,Time2)
| eval Time3=if(Time1>Time2,Time1,Time2)
For understanding time fields and conversions you can check out
https://docs.splunk.com/Documentation/Splunk/latest/SearchReference/CommonEvalFunctions#Date_and_Time_functions
https://docs.splunk.com/Documentation/Splunk/latest/SearchReference/Convert
http://docs.splunk.com/Documentation/Splunk/latest/SearchReference/Commontimeformatvariables
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