Changing the date can be dangerous because GM really expects things to be in cronological order, as far as your entries go. That said, if you change the date, but don't make them out of order (e.g. 00000003 still occurs before 00000004) you should be fine.
Just goto the entry you want (usually archives/0000000X.cgi) and edit the file in text mode. The format for the entry file is in the gm_dev_guide.html, but here it is: The entry information is stored in the first line, with the following information separated by the '|' character: 1. author = author's name (key of returned hash (case sensitive)) 2. id = numeric, never 0 3. author = alphanumeric 4. subject = title of post alphanumeric (we hope) 5. weekday = numeric (0-6, 0=Sunday, 1=Monday, etc.) 6. month = month of post numeric (1-12, no leading zeroes) 7. day = day of post numeric (1-31, no leading zeroes) 8. year = year of post numeric (format: yyyy) 9. hour = hour of post numeric (1-12, no leading zeroes) 10. minute = minute of post numeric (1-12, no leading zeroes) 11. second = second of post numeric (1-12, no leading zeroes) 12. ampm = either AM or PM
So just change the fields to be the values you need and be sure to save it plaintext.