I think the 60 days rule is for when your legal status expires in the US. You have time of two months to be in status by changing or if you are leaving the US, you have 2 months to do that. However, when you apply for OPT, you are always in status and you can delay your OPT start time as much as by 2 months after the graduation. Your status is not jeopardized any way. Sometime, it is necessary to delay the OPT start time because of the EAD processing time.

Whatever you make out of it, 16 months or 14 months, you have 2 months after your OPT is expired to be in status or leave the US. Hope this helps.