You could also use a Real Time Clock (RTC) with built-in power fail timestamp functionality such as the Microchip MCP79510. It can automatically log the time that power was lost (hour, minute, day of month & month) as well as logging when power was restored (same 4 parameters).
As an aside, if you are trying to flag a reset or shutdown, then can you not look at it from the point of view that you've just powered up, either from a reset or power being applied?