将文本附加到数据框中的列的最佳方法是？

Question

我的数据框中有一个列中包含日期数据：

> head(df$postedDate_strip2,n = 20)
 [1] NA                   "November 13, 2014" 
 [3] "September 27, 2014" "October 8, 2014"   
 [5] "December 16, 2013"  "February 8"        
 [7] "November 2, 2014"   "November 30, 2014" 
 [9] "February 18"        "August 22, 2014"   
[11] "October 26, 2014"   "January 3, 2014"   
[13] "February 3, 2014"   "October 15, 2014"  
[15] "September 12, 2014" "May 5"             
[17] "April 2, 2014"      "November 4, 2014"  
[19] "January 16, 2014"   "September 28, 2014"

所有2015年的数据最后都没有, 2015文字。是否有建议的方法只是简单地将“，2015”附加到不包含逗号的条目中？