The epoch was set very close to the start of UNIX (it started development in 1969, before epoch 0). Negative epoch dates are quite common, e.g. many people's birthdays.
In 1970, I suppose there are many people whose birthdate is before 1901-12-13?
0 UNIX epoch is 1970-01-01. So everyone between 1901 and 1970 have negative birthdays in UNIX time. E.g. Brian Keringhan's birthday is -881107200 in UNIX time.