The previous code for OS_Windows::get_ticks_usec() multiplied the tick count by 1000000 before dividing by ticks_per_second. The ticks is counted in a 64 bit integer and is susceptible to overflow when a machine has been running for a long period of time (days) with a high frequency timer.
This PR separates the overall calculation into one for seconds and one for the remainder, removing the possibility of overflow due to the multiplier.
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