TST: fix ujson's intermittently failing date unit test

Fixes #4575.

As you surmised @jreback it looks like unnecessary float arithmetic was causing the test failure. I fixed this up and in the interest of consistency removed the dependence on `datetime.now()`

Example
```python
In [8]: val = 1376774232215504000

In [9]: val / 1000
Out[9]: 1376774232215504

In [10]: val / 1e3
Out[10]: 1376774232215503.8

In [11]: val // 1e3
Out[11]: 1376774232215503.0

In [12]: val / 1000.0
Out[12]: 1376774232215503.8

In [13]: val / 10**3
Out[13]: 1376774232215504
```