Public Policy with Delayed Effective Date

I occasionally joke about the return of US secession, given the re-warmed Cold-Civil War.

A friend pointed out the parallel to the India/Pakistan partition (1947): The partition displaced between 12 and 20 million people along religious lines,[c] creating overwhelming refugee crises in the newly constituted dominions; there was large-scale violence, with estimates of loss of life accompanying or preceding the partition disputed and varying between several hundred thousand and two million.

• which also reminds me of the Israel-Palestine "unpleasantness", and the theory of a two-state solution.
  • for India/Pakistan, and for Ireland, it was called the two-nations theory
• really every change or creation of borders has this issue - people from "both sides" intermingled on either side.

(I'm also "confused" by people who "refuse" to move from (a) states that are hostile to them, or (b) towns that have been economically abandoned... cf Declining American Geographic Mobility.)

So I wondered: what if you gave everyone 20 years to prepare?

• everybody has good reasons not to do this (confusion, trust, market manipulation, etc.) But.....

What are some past public policy changes with delayed effective-dates? (or "phase-in" periods) (the key is to find cases that were actually implemented, even if a bit behind schedule, rather than most-abandoned)

ObamaCare: passed 2010, chunks not effective until 2014.

GDPR: passed 2016, effective mid-2018.

Montreal Protocol phased out CFCs: passed 1987, phased-in over decades. https://en.wikipedia.org/wiki/Montreal_Protocol

Hong-Kong Handover: 1984-1997.

Urban Planning, Regional Infrastructure projects (cough high-speed rail)