FIX: revert part of #11770
Reverts part of:

https://github.com/pydata/pandas/pull/11770

Closes:

https://github.com/matplotlib/matplotlib/issues/6365

The mistake in #11770 was missing that pandas had a 1/us not 1/s
scaled bucket.

@tacaswell any tests?

can you add a whatsnew note? (can be something as simple as compat for .....issue because of incorrect tick display with mpl < 2.0....)
 Only took 4 force-pushes to get there.

@jorisvandenbossche Added back the minimal set of constants to compute what was needed.

What sort of test do you want for this?  You merged the last one with out a test :imp: 
 can u create something that failed on the previous and now works? just maybe a check on axis ticks or something simple
 thanks @tacaswell pls verify with mpl >= 2.0 when you have a chance.
 did we have an issue where this came up on pnadas side? or was only on mpl side?

@TomAugspurger @jorisvandenbossche 
 Only on the MPL tracker.
 Sorry i took to long to get a test in.

On Fri, May 20, 2016, 10:37 Tom Augspurger notifications@github.com wrote:

> Only on the MPL tracker.
> 
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> https://github.com/pydata/pandas/pull/13131#issuecomment-220623417
 np

ideally if u can post a couple of assertions we can add something to prevente future regressions
