Yes Rahul its absolutely possible.In case I procure the entry permit well in advance before starting from Calcutta and halt at Siliguri for the first night, is it possible to reach Paro on the second day? On the second day I plan to leave Siliguri early, say by five in the morning, so that I may reach Phuntsholling by 10 O' clock and collect the required vehicle permit from the RSTA office on my way to Paro. How much time is normally required at this RSTA office, if I reach there by ten in the morning?
Say for example you are arriving in Phuentsholling at 10 AM and take 2 hours to get your vehicle permit.
So maximum by 1 PM if you are starting from Phuentosholling you can reach Paro in 5-6 hours.
As per my knowledge RSTA office takes maximum 2 hours to give you the permit.