If you have a TomTom account, and you have entered the login details on your device, you get the 5-day forecast. If you have not entered any login details, you only get the forecast for today.
The first day in the 5-day forecast is today. The date is shown.
There is nowhere to set the date, this info comes from the GPS signals.
The date shown also depends on the location. If I ask for a current weather forecast in Japan, I get tomorrows weather. Because today here, is tomorrow there (I am in the EU).
Khuon, are you asking for the weather for your current location? The weather is based on the GPS coordinate you specify. You can't get much more location aware than a GPS receiver.