Can a court order retroactive support to start from the date income actually changed, not the date of application?

Yes, a court has discretion to set the effective start date of a retroactive award to reflect when the payor's income actually changed, rather than limiting it strictly to the date the recipient formally applied to court, particularly where the delay in applying is explained by one of the recognized factors, such as the payor concealing the change or the recipient only recently discovering it. Courts are not confined to choosing only between the date of formal notice and the date of the court application; the broader retroactive analysis is meant to identify the period during which the child was actually entitled to a different amount of support.

That said, courts do weigh the practical fairness of reaching back to the exact date of an income change, especially if that predates any notice to the payor by a significant margin, against the hardship a large accumulated award might now cause. Where a payor had no reasonable way of knowing an increase might be owed and did nothing to cause the delay, courts may be more cautious about reaching back to the precise date of the underlying income change. Because the exact starting point depends on the specific facts and the reasons for any delay, evidence establishing exactly when the income change occurred is important either way.