Override - Time: custom unit from variable

Hi, I am unable to configure custom unit for the time axis from a variable (Overrides > Standard options > Unit).

When I type “time:DD.MM” (without variable), everything is OK.

When I type “time:$unit” ($unit = “DD.MM.”), then I see “$unit” on the time axis.

When I type “$unit” ($unit = “time:DD.MM.”), it has no effect.

Does anybody know where problem is?

Thanks