When the time zone changes from daylight to standard or back again on NT, the File.lastModified method for files created before the change returns a different value than it did before the change. This only happens if you have the "automatically adjust for daylight time" flag turned on on the NT date dialog. To reproduce this, write a java program that writes out the last-modified time of a file. Then change the date of your computer to after the next time change, and run the program again, on the same file, without modifying the file in the mean time. The timestamp should be the same, but it will be an hour different.