CSVSaveService defaults to using 
  DEFAULT_DATE_FORMAT_STRING = "MM/dd/yy HH:mm:ss"
if the date does not parse as milliseconds.

This is not very helpful:
- it does not correspond with the suggested default in jmeter.properties, which is now yyyy/MM/dd HH:mm:ss.SSS
- the code does not check that the format actually works.

It would be a lot better if the format were at least checked to see if it worked before proceeding to use it.

Also it ought to be possible to compare the date field with some common formats and select one that works.

Likely input formats include:
yyyy/MM/dd HH:mm:ss[.SSS]
yyyy-MM-dd HH:mm:ss[.SSS]
yyyyMMdd HH:mm:ss[.SSS]

All the above should be easy to detect unambiguously.

However, if the file uses MM/dd/yy HH:mm:ss (US format) as per the current code, it won't in general be possible to distinguish it from dd/MM/yy HH:mm:ss (as used in Europe).

In this case, the format would need to be provided as a property.