Hello Alteryx Community!
I am having issues with the multi-field tool and my expression returning null values for some rows. I am trying to calculate the duration between events in seconds and some of the rows are being calculated correctly while others are returning a null value.

I have tried to reformat the change timestamp field into a string and force a date / time format, but nothing has worked. I am still pretty new to Alteryx so I am guessing that this is an easy fix that someone has encountered before.
Thanks,
Matt