I realise this post is a year old, but for anyone else who is interested, 15/10/1582 is the first date on the Gregorian calendar, being the day after 04/10/1582 on the Julian calendar (10 days skipped to bring the year back into line when calendar was switched).
I thought this was a specific code in my data to begin with but it is likely just a default date from somewhere, similar to 01/01/1600.
Cheers