Fix a silly bit of code in date/time conversion code
(I wrote it so I can say that) which throws away
precision too soon, truncating to the nearest day.

Reference: /n/sources/patch/applied/excel-dates
Date: Mon Dec 10 14:22:37 CET 2012
Signed-off-by: steve@quintile.net

--- /sys/src/cmd/aux/msexceltables.c	Mon Dec 10 14:18:46 2012
+++ /sys/src/cmd/aux/msexceltables.c	Mon Dec 10 14:18:42 2012
@@ -196,9 +196,8 @@
 }
 
 struct Tm *
-bifftime(double num)
+bifftime(double t)
 {
-	long long t = num;
 
 	/* Beware - These epochs are wrong, this
 	 * is due to Excel still remaining compatible