There shouldn't be so much confusion on a simple thing such as IDL
REMEMBER:
Travelling west - You're going back in time - So you're Gaining a DAY
Travelling East- You're going ahead in time- So you're loosing a DAY
( EXAGGERATE it by imagining you're Superman zooming to the opposite Meridian with respect to you're current position
)
I hope all the other explanations including this one helps!
Good day!
