Difference between two timestamps

[Metrics Searcher]

Hello.

I'm trying to collect timestamps of start and stop of a cronjob vis push gateway.

All works fine if my job runs each 5 minutes. 

However, the job starts at 5 minutes of each hour, I see that the result of the different looks weird:

0:Array[1593561885,20]

1:Array[1593561900,20]

2:Array[1593561915,-3580]

3:Array[1593561930,9]

Could someone give me a clue about how to solve this issue? I probably use the wrong way to collect metrics.