Use
Timer()
Timer() returns the number of seconds elapsed since midnight as a Single value.
Usage
LotusScript rounds the number of seconds to the nearest hundredth.
Although it doesn't give you a millisecond accuracy, a Win32API call could give you that. But my experience is that the resultion is no better than 10mS anyway, so you wouldn't be much better off. See
http://www-10.lotus.com/ldd/46dom.nsf/55c38d716d632d9b8525689b005ba1c0/2133525793be2db285256b95004cc3e3?OpenDocument for examples.