You go to your current build and save on the slot you want to use (for your older save) Once you save you go and copy and paste your old save with the corresponding slot you want to use.

Lets say you save on Slot 10 But your old save is named ''file12.rmmzsave'' You need to change it to ''file10.rmmzsave'' and overwrite the newest one (the save you did)

Why is it this way? I think it is because the Save description is using Variables that are not parsed initially, (Day and Gold) so you overwriting a new file, which does have this info matching, it will Bug out the Variables info in the description (but not in game) Setting it to 0 in the Scene_Save menu and allowing the older save to work.
Worry not, If you save again after all this process, the Save description will go back to normal.

Just putting the save in there without overwriting won't cut it because the description (Day and gold) will not be parsed

Very technical description of why I think it is not working correctly, but I hope it gives you insight.