My UST Monitoring Certification is due in April, but my service technician completed it in March this year. Can my next UST Monitoring Certification be completed in April of next year?

No, the next UST Monitoring Certification must be completed by the end of March of next year.  The period between the two Monitoring Certifications cannot exceed 12 months [Refer to California Code of Regulations (CCR) Title 23 Section 2620 (e)].