TIP-1022: return modified date as millis from epoch (#502)
TIP-1022: return modified date as millis from epoch (#502)

* TIP-1022: TIP-1022: per review feedback, return modified date as milliseconds from epoch.
This requires converting it back to a date string for dx command output.

* TIP-1022: clean per code review

dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit