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Each user's profile is stored in a database and it contains fields like username, date registered, status, number of posts, &c.

One of those fields is 'date registered', which is a date/time field. When we view it on the screen it appears in a human readable format, but if you looked at the literal field content in the database you would see a long integer. For example, 1029492988 is equivalent to 16 August 2002 11.16:28....
for some reason, for those users who were caught in the glitch..the field will contain 0 which is defined by the birth date of Unix: 1st January 1970...which is why it says December 31, 1969.