manager: use portable epoch time computation

[wchargin]

Summary:
Fixes #1895 by replacing the (apparently unportable) %s format
specifier with an explicit subtraction from epoch.

Test Plan:
I don’t have easy access to a Windows machine with a Bazel/TensorBoard
setup, but I verified that this expression can be evaluated by itself:

Python 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 24 2018, 00:16:47) [MSC v.1916 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license()" for more information.
>>> import datetime
>>> dt = datetime.datetime.now()
>>> dt.strftime("%s")
Traceback (most recent call last):
  File "<pyshell#2>", line 1, in <module>
    dt.strftime("%s")
ValueError: Invalid format string
>>> int((dt - datetime.datetime.fromtimestamp(0)).total_seconds())
1551203889

Running git grep strftime.*%s returns no matches as of this commit.

wchargin-branch: windows-strftime

[nfelt]

LGTM, bonus points for good use of labels.

[mabrowning]

Sadly:

Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 27 2018, 03:37:03) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> dt = datetime.datetime.now()
>>> int((dt - datetime.datetime.fromtimestamp(0)).total_seconds())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
OSError: [Errno 22] Invalid argument
>>>

My server is set to UTC, so 0 is below the minimum time:
https://stackoverflow.com/a/45372194/381313

This works, but is gross: int((dt - datetime.datetime.fromtimestamp(86400)).total_seconds())+86400

Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 27 2018, 03:37:03) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import datetime
>>> dt = datetime.datetime.now()
>>> ts = int((dt - datetime.datetime.fromtimestamp(86400)).total_seconds())+86400
>>> dt
datetime.datetime(2019, 3, 29, 18, 23, 6, 892661)
>>> datetime.datetime.fromtimestamp(ts)
datetime.datetime(2019, 3, 29, 18, 23, 6)

[wchargin]

Yes, thank you. This is known (#2017) and fixed (#2054). If you’d like
to pull in our latest nightlies to try it out, please uninstall the
tensorboard package and then install tb-nightly.

[mabrowning]

Got it! I did go as far as checking the release notes this PR was cherry picked into... but I didn't go to check the next one. Thanks!

[wchargin]

Sure thing, and thank you for reporting it!