Hp prime lose date&hour when i change battery

[pippocalo]

thank you all for the comprehensive answers.
Indeed the explanation that RTC stops is very reasonable and I was stupid not to think about it.
In fact, if you connect the cable to the PC, the update of the date-time is automatic.
Thanks to Cyrille for her makeup.
I change the battery because my HP Prime is on for a long time every day; I bought a spare battery and I change it every month to consume them in a similar way, even i know that the li-ion batteries also kept in the drawer after 3-4 years are at the end of their life. But to me with a charge every 2 days they would last less if consumed one at a time. Alternating them, I have reasonable certainty that they will last 4-5 years.
For the future, it scares me to find "fresh built" batteries. Cellphones disappear, HP Calculators become cult objectss. I still have an HP 67 that works very well with 3 AA Ni-Mh batteries. In 10 years there will still be batteries for the HP-Prime ??

Thanks

Filippo