Problem with dates and daylight savings...

[John Taylor]

I was wondering if some one could explain this anomaly.  I wrote a program to add/subtract
dates.  For example,

./ComputeDay.py 2004/04/12 -7 %Y/%m/%d
2004/04/05

Here is the anomaly....
./ComputeDay.py 2004/04/12 -8 %Y/%m/%d
2004/04/03

I would have thought this would have returned the 4th.  I have a feeling that daylight savings
has something to do with this, as the time changed on the 3rd.
Can someone explain this?  How can I correct this problem?

./ComputeDay.py 2004/04/12 -9 %Y/%m/%d
2004/04/02



Thanks,
-John


Here is the code:

#!/usr/bin/env python2

##
## ComputeDay.py
## John Taylor, Apr 12, 2004
##

import time
import sys

############################################################################

def Usage():
	print
	print "Usage: %s [ t1 ] [ n ] [ format string ]" % sys.argv[0]
	print "Given arguments t1 and n, where n is a positvie or negative number,"
	print "return the date +/- t1"
	print """format should be a date format string, such as: %Y/%m/%d"""
	print "See the man page for strptime for formatting options"
	print
	print "Example:", sys.argv[0], "2004/03/01 -1 %Y/%m/%d"
	print
	sys.exit()


############################################################################

def main():
	if len(sys.argv) != 4:
		Usage()

	fmt = sys.argv[3]

	try:
		t1 = time.strptime(sys.argv[1], fmt)
	except ValueError, e:
		print "Error for t1: ", e
		return

	try:
		n = int(sys.argv[2])
	except ValueError, e:
		print "Error for n: ", e
		return

	oneday = 60 * 60 * 24 * 1.0
	diff = time.mktime(t1) + (n * oneday)
	diff = time.localtime(diff)

	try:
		result = time.strftime(fmt, diff)
	except ValueError, e:
		print "Error for result: ", e
		return

	print "%s" % ( result )

############################################################################

main()


-- end of code