integer time = 120; //How many seconds between checking
integer IsDay;

Check()
{
    vector sun_dir = llGetSunDirection(); //gets sun direction

    if (sun_dir.z > 0 && IsDay == FALSE) //a positive value for sun_dir.z means its daytime
    {
        IsDay = TRUE;
        //Do something here for when it is day.
    }
    else if (sun_dir.z < 0 && IsDay == TRUE) //a negative value for sun_dir.z means its nighttime
    {
        IsDay = FALSE;
        //Do something here for when it is night.
    }
}

default
{
    on_rez(integer start_param)
    {
        llResetScript();
    }
    
    state_entry()
    {
        vector sun_dir = llGetSunDirection();
        IsDay = (sun_dir.z < 0);
        Check();
        llSetTimerEvent(time);
    }

    timer()
    {
        Check(); //this calls the function to check the day / night status
    }

    touch_start(integer total_number) //only for debug - can be removed
    {
        Check();
        if (IsDay == TRUE)
        {
            llSay(0, "It is currently day.");
        }
        else
        {
            llSay(0, "It is currently night.");
        }
    }
}