Replace the CMOS battery located in the same compartment, usually, that the hard drive or SSD and in some cases, memory access, is located.
Nothing else short of a faulty system timer on the motherboard can cause that issue, and in 30 years I've never seen a motherboard system timer fail, although I'm sure it can. Always it has been the cmos battery. Once removed you can take it to get a replacement at most computer stores, camera suppliers or someplace like batteries plus.