I want to add identifier column (Date) to a time series data frame. I want
to name the "Date" column be from 1 to 30 every 1440 rows.

Say I have a data like this (I simply my actual data here):

$dat

ID       Var
1          1
2          4
3         6
4         7
5          7
6          8

How can I add identifier column (Date) from 1 to 3 every 2 rows and have
output like this:

ID       Var  Date
1          1      1
2          4     1
3         6      2
4         7      2
5          7     3
6          8      3

Thanks,

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