sorry but I don't understand your problem:
in a where condition you have to compare two similar fields: if you use " -2w@w " Splunk don't understand that you're speaking about a time in epochtime format, so you need to transform "-2w@w" in epochtime format to compare with StartTime.
To do this you have to use (as in your first example) |eval late=relative_time(now(),"-2w@w")| where Start_Time<late
So, what is the problem to use eval?
If instead you need to show in Human readable format the three dates use strftime command:
so in place of "-2w@w" I need to use $mytime.Latest$, If I go with eval late=strftime(late,"%d/m/%Y %H.%M.%S") then It does not work If value of $mytime.Latest$ is "now". It is not able to convert "now" to epochtime