[Rd] as.POSIXct.numeric change default of origin argument

[Jan Gorecki]

Hello all,

I would like to propose to change the default value for "origin"
argument in as.POSIXct.numeric method, from current missing to new
"1970-01-01".
My proposal is motivated by the fact that this is the most commonly
needed value for "origin" argument and having that as a default seems
reasonable.
Proposed change seems to be pretty safe because it is now an error.

Best Regards,
Jan Gorecki