Airtel ₹49 unlimited data with 1 day validity is active only between the time of recharge till 12 am midnight of the same day.

One would think that 1 day validity would mean validity for a total of 24 hours. This is not the case however. Say, if you did a top-up of this plan at 11:59 pm, this plan will only be valid for a total of 1 minute.