I have been trying to figure out how to set up a workflow rule for a specific field so we can track when it was last changed. 

(i.e.) 
1. We have a field called "Rating" and would like to track when it was last changed. 
2. Our goal is to be able to look at the field and track when we marked there as being no opportunity. The goal would be to routinely circle back with the accounts where the "Rating" field was not changed for years. This way we can see if an opportunity for our services arose. 
3. I don't want to users to have to enter anything in this field, just a simple lookup to when it was last changed. 

I have been trying to play around with some workflow rules and was trying to use the ISCHANGED function. Some other posts go through how to set up the workflow rules, but when I try to replicate I keep running into some sort of error. Any insights would be greatly appreciated. 

Thanks, 

1.) Does my organization even need a self-signed certificate? I have a certificate that's expiring, but when viewing the "API Client Certificate" below it shows there is no certificate is being used. (See image attached)

2.) I tried to create a new certificate that's not expiring (shown in image) but am worried it will cause downtime for our SF users. 

I have been trying to figure out how to set up a workflow rule for a specific field so we can track when it was last changed. 

(i.e.) 
1. We have a field called "Rating" and would like to track when it was last changed. 
2. Our goal is to be able to look at the field and track when we marked there as being no opportunity. The goal would be to routinely circle back with the accounts where the "Rating" field was not changed for years. This way we can see if an opportunity for our services arose. 
3. I don't want to users to have to enter anything in this field, just a simple lookup to when it was last changed. 

I have been trying to play around with some workflow rules and was trying to use the ISCHANGED function. Some other posts go through how to set up the workflow rules, but when I try to replicate I keep running into some sort of error. Any insights would be greatly appreciated. 

Thanks,