$ZDATETIME. Unix epoch Milliseconds

I am trying get the Unix time stamp  in milliseconds

set epochSecond= $ZDATETIME($ZTIMESTAMP,-2)*1000

but the result is not accurate as $ZDATETIME ignoring /truncating  the fraction of the seconds and the milliseconds calculation is not accurate with fraction seconds

for example

set epochSecond= $ZDATETIME($ZTIMESTAMP,-2)

1675830682

when its converts to the milliseconds

it became 1675830682000. not the accurate fractional seconds. 

My target system looking for the milliseconds time stamp to authenticate. 

Could you please help with steps to return the unix timestamp in milliseconds

Thanks