Formula to show Numbers for Timestamps

Hi, From cell A1 onward, I have such timestamps
0:00:00
0:00:00
0:02:00
0:03:00
0:04:00
0:05:00
0:07:00
From cell B1 onward, I need these numbers
0
0
2
3
4
5
7
Please formulate such a formula which can do this task.