system time in milliseconds

It depends on what do you want to get: a point in time, or a time span between some point. Apparently those are conceptually different and are represented by different types:
http://msdn.microsoft.com/en-us/library/system.datetime%28v=vs.110%29.aspx[^],
http://msdn.microsoft.com/en-us/library/system.timespan%28v=vs.110%29.aspx[^].

Usually, if you are talking about milliseconds or something like that, the time span is implied. Please see:
http://msdn.microsoft.com/en-us/library/system.timespan.milliseconds%28v=vs.110%29.aspx[^],
http://msdn.microsoft.com/en-us/library/system.timespan.totalmilliseconds%28v=vs.110%29.aspx[^].

Precise value is given by Total* properties; for example, not Milliseconds (representing number of full milliseconds, poor accuracy), but TotalMilliseconds. The discrepancy will be especially dramatic on hours or minutes. :-)

You can directly measure the time span, with very high accuracy: http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch%28v=vs.110%29.aspx[^].

You can obtain the actual accuracy during rumtime:
http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch.frequency(v=vs.110).aspx[^],
http://msdn.microsoft.com/en-us/library/system.diagnostics.stopwatch.ishighresolution(v=vs.110).aspx[^].

—SA

DateTime has a millisecond[^] property.
Thus, try DateTime.Now.Millisecond.