clock milliseconds, which replaces [clock clicks -milliseconds] in Tcl 8.5beta, returns the current time as an integer number of milliseconds since the epoch.
AMG: Tcl time provides the illusion that all days are 86400 seconds in length, even though occasionally a leap second is added to synchronize UTC with TAI (atomic time). After a leap second occurs, Tcl seconds are made slightly slower.
It appears [clock milliseconds] and clock microseconds are also slowed correspondingly so as to remain in sync with [clock seconds]. If they weren't, as of mid-2019 there would be a 37-second difference between [clock seconds] and [clock milliseconds/microseconds].