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.
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.