as.POSIXct before and after 1970

Can someone, please, explain the difference in as.POSIXct results
before 1970 and on and after 1970 as illustrated below.

After 1970, the use of 'EST' or "EST+5EDT' as the timezone does not
affect the result of asPOSIXct, but before 1970 on 10/28 the results
are different.

as.POSIXct('1970-10-29', tz='EST')+1

[1] "1970-10-29 00:00:01 EST"

as.POSIXct('1970-10-29', tz='EST+5EDT')+1

[1] "1970-10-29 00:00:01 EST"

as.POSIXct('1969-10-29', tz='EST')+1

[1] "1969-10-29 00:00:01 EST"

as.POSIXct('1969-10-29', tz='EST+5EDT')+1

[1] "1969-10-28 23:00:01 EST"