R中as.Date的代码说明

Question

Consider this code:

test_date<-"01-Jan-64"  
as.Date(test_date,"%d-%b-%y")
## [1] "2064-01-01"     
test_date<-"01-Jan-69"  
as.Date(test_date,"%d-%b-%y")  
## [1] "1969-01-01"

Why has 64 been read as 2064 and 69 as 1969?

Answer 1

From ?strptime

%y Year without century (00–99). On input, values 00 to 68 are prefixed by 20 and 69 to 99 by 19 – that is the behaviour specified by the 2004 and 2008 POSIX standards...

Basically don't use two digit years if there is any risk of ambiguity.