I presume you would like DevTest to do this automatically?
There are three lines to add in DEVTEST_HOME/lisa.properties.
Search that file for "datechecker".
In that section, you want to find lines like these, and add these lines to the corresponding places (I haven't syntax-checked these lines):
1st addition:
lisa.vse.datechecker.customwstimestampregex=\\d\\d\\d\\d((1[012])|(0\\d)|0[1-9]|[1-9])(([12]\\d)|(3[01])|(0?[1-9]))T(([012]?\\d)|(2[0123])):(([012345]\\d)|(60)):(([012345]\\d)|(60))\\.\\d\\d\\d[-+]\\d\\d:\\d\\d
2nd addition:
lisa.vse.datechecker.customwstimestampformat=yyyyMMdd'T'HH:mm:ss.SSSXXX
Then find the following line:
lisa.vse.datechecker.top.priorityorder=lisa.vse.datechecker.wstimestampformat
and add this line directly below it:
lisa.vse.datechecker.date.priorityorder=lisa.vse.datechecker.customwstimestampformat&\
Now restart your DevTest Workstation.
Open your service image. There is a toolbar button with tooltip of "regenerate magic strings and dates". Press that button.
DevTest scans your whole service image for strings and dates that can be abstracted. It should find your date format and do what Syed suggested. You can then change the number of days (and any other piece of the date) to match your requirements.