I want create a new colume to represent which date are in the same week. 

A data.table DATE\_SET contains Date information, like: 

```
DATA_SET<- data.table(transday = seq(from  = (Sys.Date()-64), to = Sys.Date(), by = 1))

```

For example, '2017-03-01' and '2017-03-02' are in the same week, '2017-03-01' and '2017-03-08' both Wednesday, but they are not in the same week. 

If "2016-01-01" is the first week in 2016, "2017-01-01" is the first week in 2017, the value is 1, but they are not in the same week. So i want the unique value to pecify "a same week".