Well this can easily be done using just the init.sqs and the Description.EXT
To make it easier for yourself, set your mission start time to Midnight
then add the following lines in your description.EXT, removing all previous Param1 entries
Description.EXT
titleParam1 = "DAYTIME";
valuesParam1[] = {1,2,3,4};
defValueParam1 = 1;
textsParam1[] = {"Dawn", "Midday", "Dusk", "Night"};
Init.sqs
?(param1==1):skiptime 6.2
?(param1==2):skiptime 12
?(param1==3):skiptime 18.4
?(param1==4):skiptime 0
(The skiptime values for Dawn and dusk may need to be adjusted depending what month you used, but they are fairly close)
add the following line if you use an endgame script waiting for [color]time>= param1[/color]
param1 = 1800
this will reset the gametime to 30 minutes
NB>>> make sure the endgame script doesnt activate as soon as the game starts by simply adding a [color]~10[/color] seconds line to the start of that script
Alternatively if you wanted the Param1 variable to still be used for mission length options, you could simply use Param2 instead (This is normally used for score limit)