Onset is the beginning of a condition or event. In property records, distinguish the estimated beginning from the time someone first noticed the problem.
What this means at your property
A stain discovered on Monday does not prove that moisture began entering on Monday. The source may have been active earlier or intermittently. The report should preserve that uncertainty instead of assigning a date without evidence.
What timeline is useful for an assessor?
Record the first observation, relevant weather or plumbing events and the last time the area was known to appear normal. Add photographs or maintenance records when available and label estimates as estimates.