How to convert time data into duration data, then inso seconds?

EDIT: Oops the title has a typo in it. It should be into seconds, not inso.

I have a bunch of data measuring how long it took to do something (like 1 min 30 seconds or 35 minutes 54 seconds) but when I put that into Excel using the format hh:mm:ss and selected the cell it showed 12:01:30 AM and 12:35:54 AM, respectively. I don't want the time of day, I want 00:01:30 to be the duration, not just after midnight. I tried fiddling around with the number formatting of the cells, but I can't figure it out. And then I need to convert all those durations into pure seconds, so 00:01:30 would turn into 90 seconds. I think I just put =[cell]*86400, is that right? I tried that, but since the numbers are registering as hours in the day it doesn't work, it just shows 0:00:00 in the cell with the formula in it.

Try formatting the cell as
mm:ss

Or [hh]:mm:ss if the time will go over 1 hour.

Then to convert it to pure seconds, multiply it by 86400 and format the cell as a number instead of a time.
86400 = 60*60*24, which represents 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day

I formatted the cells how you suggested, but it still showed up as a time and not duration. But I found that that didn't matter since I changed the format in the other cells to numbers and it displayed the seconds instead of 0:00:00. Thank you!

Duration is a tricky thing for excel. It doesn't really understand it 100%
Technically what it does is count hours/minuts/seconds from the date Midnight Jan 0 1900

So say 50 hours, 30 minutes would actually be considered Jan 2nd 1900 @ 2:30 AM

Formatting the cell as mm:ss will hide the time from your eyes as you look at the cell.
But it will always be there in the formula bar.