As noted, it's the day of service. You should have this date on the "proof of service" that your process server used when giving your soon-to-be ex your petition. That date triggered his time to file his response.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.
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