In Python, this is an easy exercise in using the datetime format strings:

from datetime import datetime

s = "8 March, 2017"
d = datetime.strptime(s, '%d %B, %Y')
print(d.strftime('%Y-%m-%d'))

See this table for a full description of all the format qualifiers.

Here I use the datetime.strptime method to convert your datepicker.js string to a Python datetime object. And I use the .strftime method to print that object out as a string using the format you desire. At first those format strings are going to be hard to remember, but you can always look them up. And they well organized, in the end.

I do wonder, though: might it be better to stay in JavaScript than switch over to Python for this last step? Of course, if you are using Python elsewhere in your process, this is an easy solution.

Mark Reed's advice might be best in your case. One frequently overlooked alternative for dates is the arrow module, which offers some interesting features. In this case you can do:

>>> import arrow
>>> arrow.get('8 March, 2017', 'D MMMM, YYYY').format('YYYY-MM-DD')
'2017-03-08'

As an example of a feature, if you capture that date you could format it in Russian.

>>> aDate = arrow.get('8 March, 2017', 'D MMMM, YYYY')
>>> aDate.format('YYYY MMMM DD', locale='ru')
'2017 марта 08'