Oh, you want to do his shot 2 hours later than present? That's an easy one!
You give half the regular dose at the AM time you normally give it now. Then give the second half 2 hours later so that your PM shot will be 2 hrs later than usual.
For example: half dose 8AM, another half dose at 10AM, your PM dose time is now 10PM. And you can give the full dose at the PM one. So then your new cycle is 10AM/10PM.
You could also do the splits in the PM.
That is much easier than moving in 15 minute increments. It should work fine. All it does is extend duration 2 hours ahead. I do it once a week. Check our spreadsheet, although it's not updated. I usually have to do it on Wed or Friday.