"XLConnect" packages; Excel dates read incorrectly

Hi,

   I tried to read xlsx files by "XLConnect" packages, but the dates are
one day earlier than it is supposed to be. I moved from California to
Taiwan (Eastern Asia), and it worked well in California, but not in Taiwan.
Even if I adjust my Mac time to California time zone, it gives the wrong
dates. I don't know which part of the setting (in RStudio or in my Mac?) I
should adjust. The codes and the data are attached.

   My data are on weekdays, Monday to Friday every week, but they are read
as Sunday to Thursday.

Data:
2004-01-01 (Th)
2004-01-02 (F)
2004-01-05 (M)
2004-01-06 (T)
2004-01-07 (W)
2004-01-08 (Th)
2004-01-09 (F)

The data are read as:
"2003-12-31" (W)
"2004-01-01" (Th)
"2004-01-04" (Su)
"2004-01-05" (M)
"2004-01-06" (Tu)
"2004-01-07" (W)
 "2004-01-08" (Th)



The codes are (also attached):


rm(list=ls())
library(XLConnect)
library(xlsx)

fl<-paste("allData_out3.xlsx")
a1<-readWorksheetFromFile(fl, sheet="first", colTypes="numeric")
b1<-readWorksheetFromFile(fl, sheet="second", colTypes="numeric")
a_col<-readWorksheetFromFile(fl, sheet="first")
date11<-as.Date(a_col$date, format="%Y-%m-%d")


The output:

date11

[1] "2003-12-31" "2004-01-01" "2004-01-04" "2004-01-05" "2004-01-06"
"2004-01-07"
 [7] "2004-01-08" "2004-01-11" "2004-01-12" "2004-01-13" "2004-01-14"
"2004-01-15"
[13] "2004-01-18" "2004-01-19" "2004-01-20" "2004-01-21" "2004-01-22"
"2004-01-25"
[19] "2004-01-26" "2004-01-27" "2004-01-28" "2004-01-29" "2004-02-01"

Thanks!!