dateMenu: Make clock offsets relative to local time

We recently added offsets to world clocks that represent the location's
timezone as UTC offset. However for most users, that representation is
overly technical and less helpful than the difference to their local time.

#1157
4 jobs for master in 3 minutes and 34 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Source Check
passed #271798
js_check

00:00:35

 
  Build
passed #271799
build

00:01:55

 
  Test
passed #271800
test

00:01:03

passed #271801
test-pot

00:01:03