Refer to figures.
The earth rotates from West to East => Therefore, time to the East is ahead of time to the West.

Circumnavigating the globe:

Travelling westwards around the world, you shoud set your clock:

• Back by one hour for every 15° of longitude crossed, and
• Forward by 24 hours upon crossing the International Date Line.

Traveling eastward, you must set your clocks:

• Forward by one hour for every 15° of longitude crossed, and
• Back by 24 hours upon crossing the International Date Line.

When crossing date line you gain or lose one day. When crossing from West half of the world (e.x USA) towards the East half of the world (ex. Australia) [westerly direction], you gain one day, if you were 1st of January you will automatically find yourself second of January (you gain one day in your calendar). The opposite will happen if you're travelling eastwards.

SUMMARY:
Westerly - Add a day
Easterly - Subtract a day