The Unix Epoch is in 1970, not 1900.
Unix time (also known as POSIX time or Epoch time) is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, not counting leap seconds. It is used widely in Unix-like and many other operating systems and file formats.
I use the tool at
Epoch Converter to convert variables where I can't use PHP date("U")