Typically the clock won't reset unless you lose battery power.
Check your battery terminals for corrosion and a snug fit. Give them a sturdy shake to verify they are tight. Also check the black wire (negative terminal) is firmly bolted to the body/frame sheet metal and the connection is free of corrosion. Then get the auto store to pull any pending OBD codes to see if there are any surprises waiting in the wings.
If the car is used the battery could be too. The shake test will verify the terminals themselves are secure too - if the terminal post is tight to the cable clamp but loose at the battery you need a new battery -rare, but it does happen.
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