R 如何从字符转换为as.Date

请考虑这个小数据.框架.我正在试着写一篇基于 SampleDate已格式化为.Date，但新创建的Year列 变成了一个角色，而不是日期.问题是，我如何保留年份列的格式？ try 转换为as.Date时出错. 我的代码

a <- structure(list(SampleRowID = c(109, 109, 109, 109, 109, 109), 
               SampleDate = structure(c(15803, 15803, 15803, 15803, 15803, 
                                        15803), class = "Date")), class = "data.frame", row.names = c(NA, 
                                                                                                      -6L))
glimpse(a)
$ SampleRowID <dbl> 109, 109, 109, 109, 109, 109
$ SampleDate  <date> 2013-04-08, 2013-04-08, 2013-04-08, 2013-04-08, 2013-04-08, 2013-04-08
#SampleDate is a date format

#Adding a year column doesn't keep the date format...is a character
a$year <- format(a$SampleDate,"%Y")
 glimpse(a)
 $ SampleRowID <dbl> 109, 109, 109, 109, 109, 109
$ SampleDate  <date> 2013-04-08, 2013-04-08, 2013-04-08, 2013-04-08, 2013-04-08, 2013-04-08
$ year        <chr> "2013", "2013", "2013", "2013", "2013", "2013"

try 将年份转换为as.Date格式时引发错误

A$Year&lt；-as.Date(A$Year) CharToDate(X)中出错: 字符串不是标准的明确格式