Can a variance in voltage cause the time of digital clocks in/out of microwaves 2 vary?

Not the Voltage, but the Hertz frequency..

US uses 60Hz, England uses 50hz (along with a some other countries).

if you use a converter for a digital clock made for US power to european power.. same voltage, but it will lose 10 minutes every hour.

This is of course if the clock is not set up to correct for this, i.e. atomic clock set.

Most recent digital clocks uses a quartz oscillator for time, which will not depend on voltage.

If the voltage drops enough I guess the clock stops working. I suppose (but with no evidence) that the voltage dropping just the right amount might cause the oscillator to quit but the time register to stay running, but it seems unlikely. Usually with mine they reset to 12:00 when the power goes out.