public class DialogRequirementIsNotHordeDay : BaseDialogRequirement
{
    public override bool CheckRequirement(EntityPlayer player, EntityNPC talkingTo)
    {

        if (RebirthUtilities.IsHordeDay())
        {
            return false;
        }

        return true;
    }
}

public class DialogRequirementIsHordeDay : BaseDialogRequirement
{
    public override bool CheckRequirement(EntityPlayer player, EntityNPC talkingTo)
    {

        if (!RebirthUtilities.IsHordeDay())
        {
            return false;
        }

        return true;
    }
}