Swimming 1.500 Meter freestyle nonstop. Difference between total time and the added interval time?

What does "I took interval time" mean? Have you looked at the laps, pools of the recorded activity in Garmin Connect? It might explain whatever happened

Hi flocsy, thanks for your answer.

I took intervaI time means, that i pressed the lower right bottom of my Forerunner 965 every 10 lanes, without stopping my swim.

I looked now in Garmin connect and that shows as well my problem. The total time was 24:57 minutes and the 6 interval times are alle between 3:59 an 4:08 Minutes. When i add the laps up, then the total time is 24:15 Min. Why is there this difference of 42 seconds? And what can i do, to see the correct lap or interval times?

I hope this helps to explain my question.

I'm surprised there's only 42 seconds missing, I would expect half of it to be missing, because as I remember in pool swimming the lap button switches between swim and rest mode. But apparently this is not your problem.

I'd contact Garmin Customer support in chat or email and send them the link to the activity with all the details and question.

https://support.garmin.com/en-US/

I use my 965 to swim about 4000m a week. Mostly I do 500m sets of a 25m pool. When I finish a set I press the bottom right button. Then I press it again when I start the next set. I find the 965 reliably records my laps with expected times. Occasionally I get an additional lap more or less than expected... like may once every few months. I presume I did something different to change the lap detection. 

I have never used my 965 like the OP where I just push the button every 10 laps. I think this is not the way it is meant to be used. 

Note the 965 records seperately swim (moving) time and elapsed time. Typically for me elapsed time is longer than swim time. Swim time is simply all the time while you swim not including the sections you paused it by pressing the bottom right button. Elapsed time is the time from when you press the start button to when you press it again to stop. It includes the paused time. For me I have about 5 minutes paused time in each activity.