So, just to be painfully clear here - in qRules, one of the things we can map is the actual item ID (the SharePoint List Item ID - an auto incrementing integer that every single list or library item has, whether displayed in the view or not). For updating list items, qRules uses this ID to know what to update. The mapping for the field can look like this:
<Mapping>
<FormField IsRichText="false" IsId="true" IsAttachment="false">/my:myFields/my:field2</FormField>
<SharePointColumn></SharePointColumn>
</Mapping>
So, in my case above, I'm mapping field2 in my form and have IsId set to true. In order to update my library item, prior to executing the SubmitToSharePointList command, I set field2 to the list item ID of the item I want to update.
Likely, you will use your timestamp to query the library and get the ID of the item you want to update, or you may already have it in your data connection to the library, but I wanted to clarify this in case there was any confusion.
Also - I unit tested only in preview, not in the browser, so we may or may not have challenges there if this is a browser form.