I've had experience (on the receiving end) of that inability to calculate which way the time difference goes. It was amplified in our case by our moving from several hours ahead of UK to several hours behind. Some members of our family seemed not to grasp that we had changed zones. And the knack of finding a time slot when to ring, such as before 6 p.m. our time to catch mother-in-law before she went to bed.
One of the memories that sticks is a young man in Australia who was diligently working out the time difference to/from the UK. He was doing extremely well, factoring in the fact that we in Melbourne were on (summer) Daylight Saving Time, until he asked me in all seriousness if Britain was also on Daylight Saving. I think was polite as I pointed out that it was currently mid-winter in the UK so no, they weren't on British Summer Time.