I don't think we should look into it that much, i just think it was a random date, it doesn't really mean anything. The only thing that arnav could be thinking of is once anjali delivers she'd be okay to handle the "shock" that her husband loves someone else, and maybe he plans on disclosing the truth to anjali once its "safe" for her to process the info. Maybe he thinks she might have some weird pregnancy complication if she finds out the truth abhi, so his ultimate plan might be to get his sis and the baby away from shaam and khushi once n for all. But only after anjali delivers. Other than that, i really don't think the 6 months deadline holds any meaning. Its an indian soap after all, we shouldn't expect it to be too mysterious ! :D