If your stamping has expired within the last 12 months, US Embassy in India(may be other countries too) allows you to drop your passport in a dropbox and skip in person interview for stamping.
But how is the last day counted? Turns out it would be the day the passport reaches US Embassy, not when you fill the DS-160 which is when the formula determines if it has been < 12 months since your last stamping.
From one of the forums: https://www.trackitt.com/usa-discussion-forums/h1b/1837475619/question-regarding-dropbox-us-visa-renewal-in-india
In 2016, my wife and son went to submit the document for dropbox exactly on the last day of 1 year expiration. Say visa expired July 22 2015 and they went to VFS on July 22 2016 to submit document as that was the last day of 1 year. I have filled the DS 160 a month ago.
VFS people first took their documents but in 10 mins they called back and return the document saying that the document will reach embassay next day, so it will be more than 1 year when the docs reach to embassay. I had to schedule the visa interview appointment and then they appeared for interview.