I don't know, I tried it myself, and it didn't work either. Maybe you could take a look at how the developers did it on the corresponding map?

The developers have it written like this:
SetAmbientLight(GROUND, 'c5m5_day',true , 5)
It doesn't work with or without the semicolon at the end.

In the WOE map, it's written like this:
local SET_LIGHT_GROUND = { 'Mo', 'Tu', 'We', 'Th', 'Fr', 'Sa', 'Su' }
if mod(GetDate(DAY), 3) == 0 then
local lightIndex = mod(round(GetDate(DAY)/3),7)+1
SetAmbientLight( GROUND, SET_LIGHT_GROUND[lightIndex], nil, 0 )
end

It still doesn't work. It says, "value was nil when getting global with name "round""
"attempt to call a nil value"