I don't know if anyone else has discovered this yet, but I haven't seen it posted, so I'll mention it here.
I started a sorta long gem forge towards the end of a rebirth (4 years game time), and ran out of money to speed things along, so I decided to let it sit. When I started that next rebirth, I noticed that it said it was 3 years later. On a hunch, I checked my gems again and it said only 1 year remained. Looks like the time it says happens between rebirths actually counts toward the gem forge time!