[dmak]

Actually, the point is to manage expectations. Instead of saying I'll be there in 5 minutes, it would be better to say I am going to be X minutes late. There's lying to hide your business, but there's also lying and being inconsiderate of other people's time.

[Squarel]

Or, it could be your best estimate, based on current traffic conditions or walking speed.

I always underestimate how long it will take,this does not mean I am lying, it means I am bad at guessing how long it takes me to walk/drive/sit in a train somewhere

[lmm]

In which case surely your friend getting an automatic, accurate version of your position from facebook would be better for both of you?