Setting this environment variable sets the value in setupVars.conf, but this is only used in chronometer (as you have seen) and PADD (also - use PADD, it's much sexier than chronometer)
For the web interface, the setting was moved to a per-browser setting alongside some others in this PR:
https://github.com/pi-hole/AdminLTE/pull/1422
Some further discussion around the subject can be found: