Will an inter post transfer extend your time in the army?

Nothing except a court martial can involuntarily extend your ETS. Depending on where you are transferring to you may be required to extend or reenlist to meet the SRR for your new assignment

Yeah, enlisted contracts are very cut and dry, if your time is going to be extended or cut short, there will be paperwork with YOUR signature on it agreeing to it. If you don't have to talk to your Reup NCO, then no worries on adding anything.

That is just an intra post transfer, all it does is change your uic. Nothing that will affect you ADSO or ets date