If a power loss occurs for less than 24 hours and the time of day flashes, this indicates the battery is depleted. The time of day should be reset and the non-rechargeable battery should be replaced.
The battery is a 3 Volt Lithium Coin Cell type 2032 and is readily available at most stores. To access the battery, remove front cover and extract from it's cradle. Slide in the replacement battery and replace front cover.