Legal Question in Business Law in California

The answer depends on what you mean, so we need more information to answer your question.

There are different reasons for filing a proof of service with the court, or for having one prepared for documents that are not filed with the court.

For example, if there is a hearing, and you were required to give notice of the hearing, the court expects the proof of service to be filed so that the court will know the other party received proper notice of the hearing. It may or may not be an issue if you provide it to the court one day late. If the other party shows up, probably not a big deal.

If you served written discovery responses late, resulting in your proof of service being late, that probably isn't a big deal either.

If the court gave you a certain time by which to serve a defendant and file a proof of service, this may or may not be a big issue, depending on whether an Order to Show Cause re Proof of Service was set. Usually, if you show up and let the court know that it has been filed, no action will be taken.

If none of these scenarios apply, we need more info.

You are going to have to provide more information. I can't tell if you are late returning a proof of service on a summons and complaint, late serving a motion or opposition to a motion, or that your proof of service conflicts with the postal meter or cancellation date on the envelope that as mailed.