I'm making a location where I want to have always the same time of day. Is it possible to set the particular time in an interior cell by scripting?

Or I'll have to use a workaround, e.g. create a door which opens only at the desired time (a la door to the Cavern of Incarnate), and then execute command "Set timescale to 0" by script (and then reset timescale to the usual 10 once the player leaves the cell)?

I would use something like

float oldTimeScaleset oldTimeScale to timescale

to store/restore original timescale, standard value is 30 but any user can have it customized
also, I would avoid setting timescale to 0, it could crash mods dividing by timescale,
better something like
set timescale to 0.001

you can change game time with e.g. 14:30
set gamehour to 14.5