合并不规则的时间序列数据

Question

我有data_VOC_A和data_VOC_B是小时数据集，但是它们是不规则的，所以当我合并两个数据集时，我想填充空白单元格NA。

  #data frames


        data_VOC_A <-structure(list(Date = structure(1:2, .Label = c("09.09.2011 21:54", 
"09.09.2011 22:59"), class = "factor"), hexane = c(0, 0), benzene = structure(1:2, .Label = c("0", 
"4.4"), class = "factor"), toluene = c(2.2, 2.6)), .Names = c("Date", 
"hexane", "benzene", "toluene"), row.names = 1:2, class = "data.frame")

    data_VOC_B<-structure(list(Date = structure(1:2, .Label = c("09.09.2011 21:54", 
"09.09.2011 22:59"), class = "factor"), ethane = c(14.4, 868.9
), propane = c(6.4, 32.1), isobutane = c(1.7, 2), n.butane = c(3.1, 
3), isopentane = c(5.6, 3), n.pentane = c(1.4, 2.4)), .Names = c("Date", 
"ethane", "propane", "isobutane", "n.butane", "isopentane", "n.pentane"
), row.names = 1:2, class = "data.frame")

#merge data frames 
d2<- merge.data.frame(data_VOC_A,data_VOC_B, by='Date', all=TRUE)

合并数据框（d2）的时间轴存在问题。