Compatibility issue mac/PC on date format - r

Question

Everything works at work on my PC but at home with my mac I meet a problem
I wrote my data on Excel,
it formats date dd/jj/yy even if I write dd/jj/yyyy but it keeps in memory the way I wrote it (dd/jj/yyyy)
I save the file as a CSV and read it in a data.frame
here is the problem :
data$ddn
[1] 29/11/58 25/07/64 25/09/67 03/01/82 15/05/58 29/07/78 22/03/69 23/01/60 15/12/60 16/06/64
[11] 10/12/60 23/08/78 13/04/67 29/11/59 25/09/56 10/10/87 22/06/60 21/06/76 01/11/63 08/07/69
[21] 22/05/52 06/05/69 04/03/64 08/04/75 09/03/54 22/04/69 29/04/71 18/03/79 14/06/71 03/06/60
71 Levels: 01/11/63 01/12/40 02/07/48 03/01/82 03/05/68 03/06/60 04/03/64 05/01/62 ... 31/07/70
> class(data$ddn)
[1] "factor"
data$ddn<-as.Date(data$ddn,format="%d/%m/%Y") (this syntax works perfectly on my PC)
data$ddn
[1] "0058-11-29" "0064-07-25" "0067-09-25" "0082-01-03" "0058-05-15" "0078-07-29" "0069-03-22"
[8] "0060-01-23" "0060-12-15" "0064-06-16" "0060-12-10" "0078-08-23" "0067-04-13" "0059-11-29"
[15] "0056-09-25" "0087-10-10" "0060-06-22" "0076-06-21" "0063-11-01" "0069-07-08" "0052-05-22"
[22] "0069-05-06" "0064-03-04" "0075-04-08" "0054-03-09" "0069-04-22" "0071-04-29" "0079-03-18"
[29] "0071-06-14" "0060-06-03"
data$ddn<-as.Date(data$ddn,format="%d/%m/%y")
data$ddn
[1] "2058-11-29" "2064-07-25" "2067-09-25" "1982-01-03" "2058-05-15" "1978-07-29" "1969-03-22"
[8] "2060-01-23" "2060-12-15" "2064-06-16" "2060-12-10" "1978-08-23" "2067-04-13" "2059-11-29"
[15] "2056-09-25" "1987-10-10" "2060-06-22" "1976-06-21" "2063-11-01" "1969-07-08" "2052-05-22"
[22] "1969-05-06" "2064-03-04" "1975-04-08" "2054-03-09" "1969-04-22" "1971-04-29" "1979-03-18"
[29] "1971-06-14" "2060-06-03"
R choose to put 19 or 20 before the date and I do not know why
And if I modify the original data (cell format : text or standard instead of date) it 29/11/58 becomes 20056 (again, I am perplexed).
I thought it was a EXCEL problem but the CSV which works with R on my PC doesn't work on mac
How to correct this R compatibility problem between PC and mac?
Thanks.