Thanks everyone (and especially Sqiddster)!
I knew there must be a system expression to do that but I couldn't find it anywhere. I had overlooked the System Reference section of the manual <img src="smileys/smiley9.gif" border="0" align="middle" />
I've never needed to use Math.floor before, otherwise it would have been blindingly obvious. How embarrassing.
Yes. Anyway. Thank you, chaps! <img src="smileys/smiley42.gif" border="0" align="middle" />