Anyone used blusmart while arriving at Delhi airport?

I was wondering how does their pre-booking service work. I will be arriving Delhi airpot in couple of days. And was thinking to pre book a blusmart cab.

How does the timing work? The app insists on a specific time. How flexible are they in terms of slight delay like 20-30 mins? I can never know exact time as getting luggage is always a variable ( provided that flight is on time ).