anonymous user-1291,
To meet your requirement, use the calculated column rather than conditional formula, check as following:
Create a new column named DateCal, choose the Calculated type, then fill in the formula =IF(StatusTest="Complete",TODAY(),"")
to check another choice column value, return as date and time:
Since this column is a formula-based column, it is a read-only field by default:
Result for your reference:
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