I have a date column and I want to just use the information in this one column

```
      Date 
2020-01-05 
2020-01-30  
2020-01-20 
2020-01-10  
2020-01-15 
2020-01-30 

```

I create a new column

```
df$3to1_difference

```

The function i want to create gives me this result. I would like to have the third column subtract the first column

```
    Date | 3to1_difference
2020-01-05  N/A
2020-01-30  N/A
2020-01-20  15
2020-01-10  -20
2020-01-15  -5
2020-01-30  20

```