麻烦请帮我看一下，数据是随访数据，我想新生成一个时间变量，表示末次随访时间。Freq表示随访次数，报告日期n表示第几次随访的时间。以下是我写的程序，总是出错，是不是因为赋值不是一个固定值的原因？请教大家，应该怎么写这个命令？多谢啦！
temp$time<- within(temp,{
                   time<-as.POSIXct(报告日期)
                   time[Freq==2]<-as.POSIXct(报告日期2)
                   time[Freq==3]<-as.POSIXct(报告日期3)
                   time[Freq==4]<-as.POSIXct(报告日期4)
                   time[Freq==5]<-as.POSIXct(报告日期5)
                   time[Freq==6]<-as.POSIXct(报告日期6)
                   time[Freq==7]<-as.POSIXct(报告日期7)
                   })

结果出错信息：
Warning messages:
1: In NextMethod(.Generic) :
  number of items to replace is not a multiple of replacement length
2: In NextMethod(.Generic) :
  number of items to replace is not a multiple of replacement length
3: In NextMethod(.Generic) :
  number of items to replace is not a multiple of replacement length
4: In NextMethod(.Generic) :
  number of items to replace is not a multiple of replacement length
5: In NextMethod(.Generic) :
  number of items to replace is not a multiple of replacement length
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FROM 210.75.207.*