I would track it all as though it happens in one day - you'll make it up on the other side when you return to Guam (ie - on the way there you'll gain a day and have a really heavy calorie day, on the way back you'll lose a day, so probably a day with no, or very few, calories
. There is a time zone setting in the 'My FatSecret'/"My Toolbox"/"My Account" section, where you can update your timezone to your current location.
Unless of course you're moving to Hawaii? In which case, you probably just need to live with the fact that you'll have one day with a lot of calories. Know in yourself that you lived the hours to burn them off and move on
I also offer the following unsolicited advice: Please don't eat the plane food (unless you're flying First Class on Emirates, that food is worth it). Pack your own food and take it on with you. This has the benefit of the fact that you can eat it whenever you please (rather than waiting for people to decide you should be hungry and feed you), and it's way tastier! I know it's yet another thing to do before you leave, but I swear it is worth it. I travel a lot with work and always take carrots, broccoli, and homemade hummus on the plane with me, as well as something more like a meal - tabbouleh in a little container, or rice paper rolls also travel well, as does a quinoa salad. This was even before I was dieting for weight loss. If you don't have time for that, stop in the airport before you leave and grab a take out salad or sandwich somewhere. Life is just too short to waste those precious calories on plane food!