The clock does not reset as long as you have had an HSA account with a nonzero balance within the 18 months prior to the establishment of the new HSA account. If you established the new HSA within that period, you can reimburse yourself from the new HSA for any medical expense incurred after the establishment of the previous HSA.
See IRS Notice 2008-59 Q&A-41: <a rel="nofollow" target="_blank" href="
https://www.irs.gov/irb/2008-29_IRB">https://www.irs.gov/irb/2008-29_IRB</a>
