I've been tried to figure out what the format of your date is. I think that number is the number of milliseconds since 1/1/1970. This system is called UNIX time, which normally counts the number of seconds, no milliseconds, but it is just a conversion needed.
Check this site: Epoch Converter - Unix Timestamp Converter. It is a timestamp converter. I think you will find it interesting. If you are using Excel* to open the .csv file you should apply this formula to the number you are getting:
=([Number in Milliseconds]/86400000)+25569
I hope you find this information helpful.