Date change when photo was taken before 1970

[Loekarin]

I have digitalised negatives from older photo's. This includes photo's from my parents and grandparents and older familymembers. The original photo's were taken a long time ago. (Long) before 1970.

In a previous topic Michel038 said:

"For dates, use Metadata>Change timestamp screen...
You can adjust the 3 Exif dates as you wish, but the "File" dates MUST stay later than 1970 01 01 . (This date equals to 0 in operating systems)"

So, that brings me to the questions:
-How to add or adjust the time stamp, and/or add the date the original photo was taken, when this is before 1970-01-01?
-And why equals 1970-01-01 to 0 in operating systems?
(NB people can also be born before 1970-01-01, and it's possible in most systems to add the date of birth of any date).

[michel038]

In "Change timestamp" screen, choose the dates to update :
- The three exif dates can accept any date like 1890-01-29
- Let File created and file modified dates unchanged (unmark these checkboxes)

When displaying or sorting photos, use an exif date instead of file system dates. Most of fields that are used in XnViewMp can be selected in large lists.

A file cannot have been created when computers did not exist