why non network (no NTP) computer does not keep accurate time

what causes a computer to not keep accurate time? case in point a Dell server, r540/640/740 type having RHEL-8.10 installed, no network connection. It is standalone, there is no NTP or chrony in use, and time had been manually set in Linux via date mmddhhmm to the current time per my watch which I manually set looking at *time.gov*. After sitting powered on an idle for maybe month, time on the standalone server is off by 30 minutes. Why? Is it a hardware issue, or is it a Linux [software] issue? My 20 year old battery powered wristwatch, and my 1980's era GE bedside alarm clock keeps way more accurate time. hwclock --show --verbose hwclock from util-linux 2.32.1 System Time: 1745946822.667986 Trying to open: /dev/rtc0 Using the rtc interface to the clock. Last drift adjustment done at 0 seconds after 1969 Last calibration done at 0 seconds after 1969 Hardware clock is on UTC time Assuming hardware clock is kept in UTC time. Waiting for clock tick... ...got clock tick Time read from Hardware Clock: 2025/04/29 17:13:43 Hw clock time : 2025/04/29 17:13:43 = 1745946823 seconds since 1969 Time since last adjustment is 1745946823 seconds Calculated Hardware Clock drift is 0.000000 seconds 2025-04-29 13:13:42.873561-04:00