The system is 10 years old and it is possible that the CMOS (coin cell) battery is bad. Try changing the battery and be sure it is a new battery not one that may have been hanging around in a drawer. I am not sure which form factor system you have but this link is for the MT. Another possibility is that though the PSU seems to test good it may be bad. Only installing another PSU would confirm yours is working or not.
-Forum Member since 2016 -I do not work for Dell -A user helping other users -If you like an answer please click on thumbs up -If a reply is a solution to your problem please click on Accept as Solution Gaming on a home built ASUS B450-F Gaming with Ryzen 5 2600X Real work on an Inspiron 15 5575 and Latitude E5430