Just to be sure, ```rng``` is already defined in common_setup; it is best practice to redefine it?

vbench run:
```
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------
datetimeindex_converter                      |   0.8059 | 411.0680 |   0.0020 |
-------------------------------------------------------------------------------
Test name                                    | head[ms] | base[ms] |  ratio   |
-------------------------------------------------------------------------------

Ratio < 1.0 means the target commit is faster then the baseline.
Seed used: 1234

Target [dc95c4c] : Added vbench performance benchmark for speed up of DatetimeConverter.
Base   [4216178] : Merge pull request #5505 from jsexauer/fix5505
```