I just thought of an even more complicated way!
An object of known weight is slid to the edge of a table and sent on a free fall next to a vertical ruler.
Fall is an acceleration (9.8 m/s squared), but its quite stable.
The object's blur can be measured against the ruler. With the edge of the table visible in the image we can find the acceleration, and thus the time elapsed.
I really should be asleep now.
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