Some months ago I put a question on stackoverflow.
http://stackoverflow.com/questions/30026790/editing-default-date-format

When you are overhauling defaults for 2.0 - is there any considerations for changed date defaults or rc-params using locals or ISO style?

I read about (not really) related topics here:
#4808
#4809

As stated at stackoverflow editing the files didn't work as aspected.
If it is not in scope of your work some help at stackoverflow would be nice.

Text at stackoverflow:
locale is "none"; resulting in "american style" dates (Apr 25 2015) instead of iso style (YYYY-MM-DD)
On smaller scales (days or lower), I need to get date information printed to some where (thought of secound and secound to last ticks): This one should be easy when the above is working...
Solution 1: Adjusting the dates in every single plot using: xaxis.set_major_formatter(matplotlib.ticker.FuncFormatter(format_date))
or
myFmt = matplotlib.dates.DateFormatter('%Y-%m-%d')
xaxis.set_major_formatter(myFmt)
--> I need to add this lines to every plot (static/editor and interactively saved ones)
Solution 2: editing the files of matplotlib (dates.py ticker.py); I found the AutoDateFormatter but changed defaults do not apply. My desire is something like this (from some years ago), but that trunk version looks like my locale one: Matplotlib autodatelocator custom date formatting?
--> Couldn't examine where and which "fmt" is used