Hi @whitegerbera,
Please make sure you have a excel table saved in your Onedrive for business and with a "Task Title" and a "Completion Date" column as below:
And please make sure each Task Title would have a same and unique task title in the planner task.
Then you could create a flow as below:
The expression in the "Completion Date" column as below:
formatDateTime(utcNow(),'yyyy-MM-dd')
When a planner task which's title is "task1" is completed, the flow would run successfully as below:
The excel table would be updated as below:
Best regards,
Community Support Team _ Alice Zhang
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