Hello @Sidkarora - If your focus is on a particular status only, let's say when the task status is set to "Complete". You'll have to create a calculated field and apply the formula. More details below:
Calculated Field: Date of Completion (set as Date or DateTime format)
Formula: IF({status}="CPL",IF(ISBLANK({DE:Date of Completion}),$$NOW,{DE:Date of Completion}),{DE:Date of Completion})