According to Schedule CMS instructions at <a rel="nofollow" target="_blank" href="
https://www.mass.gov/files/documents/2019/02/12/dor-2018-inc-sch-cms.pdf">https://www.mass.gov/files...>, it's the tax period end date in which the credit originated -- assuming you're filing your 2018 return, this'd be 12/31/2018.
