Dataframe: 

```
Number Time 
1       10:25:00
2       10:35:15
3       10:42:26

```

For each number in the data frame I want to subtract Time, for example:

```
Number 1 = 10:25:00 - 10:35:15
Number 2 = 10:35:15 - 10:42:26

```

My code:

```
for (i in df$Number) {
    for (j in df$Time) {
        subtime <- df$Time[j] - df$Time[j+1]
      }
}

```

This code only results in NA