Add functions `PERIOD_ADD` and `PERIOD_DIFF`.#130

Merged

Yury-Fridlyand merged 2 commits intointeg-datetime-period-functionsfrom dev-datetime-period-functions

Oct 19, 2022

+487-145

Commits on Oct 7, 2022

Add functions PERIOD_ADD and PERIOD_DIFF.
Yury-Fridlyand
committed

Commits on Oct 18, 2022

Minor test improvements according to PR feedback.
Yury-Fridlyand
committed