Diagnosis
The clock has no backup power source. This is the expected behavior when AC power is completely removed.
How to Fix It
This is not a malfunction. To avoid resetting, do not turn off the dedicated 240V circuit breaker for the range. Use the control panel to turn the appliance on/off.
If That Didn't Work
If the clock resets during normal use without a power interruption, the electronic control board may have a faulty memory circuit. This is a less common failure.
Heads Up
After any full power loss, you must reset the clock. The oven's timer and delayed start functions will not operate correctly until the clock is set.