The last major change to timekeeping in the US was in 2007, when the start and end dates of daylight saving time were adjusted.

More recently, proposals including the Sunshine Protection Act and similar bills in various states, have sought to change official time standards. These have inspired other Manifold questions such as Will the US make daylight savings time permanent in 2023?NO‌. Will any of these proposals be implemented in the 2020s? In what year?

"Significant" means that it affects the entirety of a populated subdivision, that is, a state, the District of Columbia, or a territory with a permanent population. (For Arizona, this may be just the part of the state outside the Navajo Nation and Hopi Reservation.) This means, for example, redrawing time zone boundaries so that they don't pass through certain states wouldn't count. (Note, however, that the change need not be a "permanent daylight savings time"; this could also include a state adopting permanent standard time as provided for in current federal law.)

This question resolves to the first year such a change takes effect, not when legislation is passed. The change may be reverted later, but it must actually have taken effect before the reversion.

Clarification: "Actually take effect" means that there is a realized difference between the official time without the change and the time with the change. For example, if a state decides to adopt permanent standard time in the year following the law's passage, this would actually take effect in March of that year when daylight saving time would have started without the new law. If the law were repealed before then, then the change would not have actually taken effect.

To avoid some potential ambiguities, years will be defined in terms of UTC-5, the current standard used for Eastern Standard Time.

I may trade in this question as usual.