I have a dataset in R sorted in descending order by timestamp in column A (mm/dd/yyyy hh:mm 24h). I am looking to create in column B a running count of timestamps in column A occurring since midnight for each unique day. I would like to produce the following outcome below:

Example:

```
| Column A        | Column B |
|-----------------|----------|
| 9/26/2019 23:29 | 4        |
| 9/26/2019 17:29 | 3        |
| 9/26/2019 8:29  | 2        |
| 9/26/2019 1:10  | 1        |
| 9/25/2019 15:21 | 4        |
| 9/25/2019 10:19 | 3        |
| 9/25/2019 5:29  | 2        |
| 9/25/2019 0:29  | 1        |

```

I have solved this in Excel by using a countif function for the unique mm/dd/yyyy value in Col A. I have not been successful to find an R equivalent.