Architect DateTimeDiff returns null for date comparison

We are trying to compare two date strings in an Architect expression to filter records. The documentation suggests using DateTimeDiff, but it consistently returns null. We have verified the input format is ISO 8601.

DateTimeDiff('{{contact.data.created_date}}', '{{current_time}}', 'day')

Is there a specific parsing requirement we are missing? The logs show no syntax errors.