On my Mk2 I had the Trip A miles and mpg set to reset when refuelled, and the elapsed time was since engine start.
This suited me very well. I liked to see the elapsed journey time, and I could compare the displayed mpg with the actual tank fill mpg.
On my Mk3, AFAICT, the customisation option for elapsed time is now linked to the Trip A miles and mpg (p133 in manual). So its says how many hours since last fill, which is not very useful.
I could juggle about with different settings for Trip A and Trip B, but it seems a backward step to me.