Automations can’t currently use conditional logic inside the automations themselves. All such logic needs to be setup as part of the automation trigger system, usually by using one or more formula fields.
You didn’t mention what date you want to use as part of the update, so I’m going to assume that it’s the date that the field in question was modified. As for the field in question, I’ll refer to it as {Target Field} going forward.
You’ll need to add a “Last modified time” field named {Last Modified} that only tracks changes to {Target Field}. Also add a date field named something like {First Modified} where this one-time-only date change will be stored. Finally, add a formula field named {Capture Update Date} with this formula:
AND({Target Field}, NOT({First Modified}))
That will output a 0 as long as {Target Field} and {First Modified} are both empty. Once Target Field contains anything, it will output a 1. Use that to trigger an automation using a “When record matches conditions” trigger, with the condition being that {Capture Update Date} is set to 1. Add an “Update record” step that takes the value from {Last Modified} and inserts it into {First Modified}. Once that insertion is done, the formula in “Capture Update Date” will output a 0 again, and the automation won’t trigger again for that record.
