Of course it is absolutely normal that switches without a battery-buffered clock will restore the date & time on a reboot if it was set manually. Most switches/routers/aps do that this way, since usually those devices are up and running 24/7. To have the switch set the date & time automatically after reboots, use NTP.

Also, NTP time services don't need necessarily an Internet connection, but having Internet makes the NTP setup of the switch or other devices as easy as just specifying the name of one or more NTP servers. If you don't have or don't want to use an Internet connection, you could set up a NTP time server inside your LAN, say on some PC or file server which has battery-buffered time (and probably a DCF-77- or GPS-controlled clock if you want the exact time).