I have two data tables. The first, DT1, is grouped by ID. It has a Week column that contains an instance of every consecutive week in a large time frame for every ID. It also contains a Units column. It looks something like this simplified version:

```
ID        Week        Units
A         1/1/2019    5
A         1/8/2019    9
A         1/15/2019   0
A         1/22/2019   1
B         1/1/2019    1
B         1/8/2019    32
B         1/15/2019   2
B         1/22/2019   6
C         1/1/2019    0
C         1/8/2019    0
C         1/15/2019   8
C         1/22/2019   3

```

The second table, DF2, has a time range window for every ID. This is represented by a start date column and a stop date column. It looks something like this simplified version:

```
ID         Start Date         Stop Date
A          1/1/2019           1/8/2019
B          1/8/2019           1/22/2019
C          1/8/2019           1/15/2019

```

I would like to calculate the sum of the units column for every date range/buyer combination in DF2. My desired output would be:

```
ID         Start_Date         Stop_Date       sumUnits
A          1/1/2019           1/8/2019        14
B          1/8/2019           1/22/2019       40
C          1/8/2019           1/15/2019       8

```

Is there a way to calculate this type of sum in R?

I have referenced the article, [In R: how to sum a variable by group between two dates](https://stackoverflow.com/questions/44345651/in-r-how-to-sum-a-variable-by-group-between-two-dates)
, as well as attempted the interval function and a non-equi join.

```
DT1[DT2[DT1, sum(x), on = .(Units, Week>= Stop_Date, Week<= Stop_Date),
                by = .EACHI], newvar := V1, on = .(Units, Start_Date 
                =Week)]

```

It returns the message, "Week" not in `i`. How do I solve the problem?