Tim Colling posted on Google+ and it was also added to the Local Search Forums that Google My Business is now asking business owners to set the date they first opened up their business.
Here is a screen shot from Tim:
I tested it myself by typing my company name into Google search and since I was signed in to the Google account that manages my Google My Business listing, I also got the question.
It asks for the month and year but you can also specify the exact date, if you wanted to.
I don't know where Google will use this information but they are now asking for it.
Some are saying there was a way to specify the open date in the old user interface but it was kind of hidden. I am not sure but now they are asking for this information in a very bold way.
Forum discussion at Google+ and Local Search Forums.