what to do when a judgment had been executed but respondent learned after 2 mosthat date of separation was antedated by six mos

In Illinois, the "date of separation" ISN"T the physical separation of the parties, nor is it defined merely by the filing of the Petition. While judgment was "executed" in just 2 weeks after filing, it is still possible that the actual "date of separation" was in excess of 6 months.
Also in Illinois, if Judgment is entered by Agreement, BOTH parties have to execute a stipulation and waiver indicating the parties have been living "separate and apart" for a period in excess of 6 months and waive the remainder of the 2 year separation period.
The respondent will have a hard time explaining why he/she signed an affidavit as to the separation period but now is claiming the separation period wasn't met.