Gabby, when I first got here 5 years ago I tried that. The officers at the Western checkpoint didn't want to give me 30 days on re-entry from Guat. This was standard procedure in other countries, in South America, etc. When you come in tell the officer where you're going to stay. You can then go to the nearest Immigration office, Belize city, Ambergris caye, etc, once 29 days have passed. Belmopan only if you're staying in Cayo.