如何根据另一个数据表中的条件从数据表中删除行

I have 2 data frames:

我有2个数据框:

master = data.table(MasterTimes= as.POSIXct("2015-01-01", tz = "GMT") + seq(1,100,1))
mydata = data.frame(MyTimes = as.POSIXct(c("2015-01-01 00:00:18","2015-01-01 00:00:54","2015-01-01 00:00:48","2015-01-01 00:01:10","2015-01-01 00:01:05"),tz = "GMT"))
master = dat