Depends on how long the battery been out of the cell phone or how drain is the battery inside the cell phone. On my other equipments, without a battery installed, the internal battery will provide up to 48hrs before you lose all settings. The int battery gets charged by the ext battery.
Just look at your motherboard. See that watch battery? It's there for a reason. to tell time. LOL!!!
Now, if the watch battery is taken out, you will have to adjust the date/time/bios settings every time you power off the computer.
Hrmm. What do you mean by the bolded? The battery is merely out of the phone for a few seconds for a battery swap. The battery is not dead yet. It's very close to it. (Close meaning I've gotten the little warning that I need to charge my battery.)
Or for example, I dropped my phone. I have a new cover for it which isn't as durable as the last one. The battery popped out. When I put it back in, the date and time had reset to 2000.
I've never had this issue before. Just in the last month or so.