diff --git a/libc/tzcode/difftime.c b/libc/tzcode/difftime.c
new file mode 100644
index 0000000..da77977
--- /dev/null
+++ b/libc/tzcode/difftime.c
@@ -0,0 +1,65 @@
+/*
+** This file is in the public domain, so clarified as of
+** 1996-06-05 by Arthur David Olson.
+*/
+
+#ifndef lint
+#ifndef NOID
+static char	elsieid[] = "@(#)difftime.c	8.1";
+#endif /* !defined NOID */
+#endif /* !defined lint */
+
+/*LINTLIBRARY*/
+
+#include "private.h"	/* for time_t, TYPE_INTEGRAL, and TYPE_SIGNED */
+
+double
+difftime(time1, time0)
+const time_t	time1;
+const time_t	time0;
+{
+	/*
+	** If (sizeof (double) > sizeof (time_t)) simply convert and subtract
+	** (assuming that the larger type has more precision).
+	** This is the common real-world case circa 2004.
+	*/
+	if (sizeof (double) > sizeof (time_t))
+		return (double) time1 - (double) time0;
+	if (!TYPE_INTEGRAL(time_t)) {
+		/*
+		** time_t is floating.
+		*/
+		return time1 - time0;
+	}
+	if (!TYPE_SIGNED(time_t)) {
+		/*
+		** time_t is integral and unsigned.
+		** The difference of two unsigned values can't overflow
+		** if the minuend is greater than or equal to the subtrahend.
+		*/
+		if (time1 >= time0)
+			return time1 - time0;
+		else	return -((double) (time0 - time1));
+	}
+	/*
+	** time_t is integral and signed.
+	** Handle cases where both time1 and time0 have the same sign
+	** (meaning that their difference cannot overflow).
+	*/
+	if ((time1 < 0) == (time0 < 0))
+		return time1 - time0;
+	/*
+	** time1 and time0 have opposite signs.
+	** Punt if unsigned long is too narrow.
+	*/
+	if (sizeof (unsigned long) < sizeof (time_t))
+		return (double) time1 - (double) time0;
+	/*
+	** Stay calm...decent optimizers will eliminate the complexity below.
+	*/
+	if (time1 >= 0 /* && time0 < 0 */)
+		return (unsigned long) time1 +
+			(unsigned long) (-(time0 + 1)) + 1;
+	return -(double) ((unsigned long) time0 +
+		(unsigned long) (-(time1 + 1)) + 1);
+}