Need to capture User Info when values are changed

I am using SharePoint 2013. While I feel like I am an advanced SharePoint end user, I am fairly new to Nintex forms and workflows. I am not sure how to explain what I need to do but I’ll try….Here is my issue: I have a form where multiple offices are required to update the column for their office. I have alerts and versioning already set up so I know who made a change and what the change was. However, now management wants to know when and who updated each columns. There are 15 offices in my organization so I set up 15 drop-down columns (one for each office). I also added a comment box for each drop-down column (one for each office) and set it to append. I told everyone to make sure to enter something into the comments box (e.g. name) so it would automatically do a time stamp and leave the current users information when the form is submitted. However, people being people, I can’t get everyone to comply with my instructions. What I would like to do is set up some sort of trigger for each of the 15 drop-downs that would kick off a workflow to capture the current user and the current date and then update another column with that information. Sounds easy enough, but I can’t figure out how to get it to work. I was hoping that there was some sort of an “on change” function but can’t find anything like that. I would like to set up a condition where If the value for a column is changed, from null to anything else, to update another column with the Current User and Current Date. I would have to do this for each of the 15 drop-downs. I’m no coder so I’d rather not have to write a bunch of code. Thanks!