研究指出，数十亿年前地球自转更快，一天并非 24 小时；在约 10 亿年的一段时期内，一天可能长期维持在约 19 小时，原因是海洋潮汐、上层大气潮汐与月球引潮力之间达到平衡。一般情况下，月球造成的潮汐摩擦会缓慢消耗地球自转能量，使日长随时间逐步增加；NASA 的概述估计这种效应约使每天每世纪变长 0.002 秒。

Ross Mitchell 团队汇整了来自沉积岩的数十笔古日长估计，时间覆盖约 25 亿年，并利用「旋回地层学」读取反映轨道与自转周期的重复层理。结果显示地球自转史并非平滑减速，而是存在长时间的平台期：其中最醒目的平台发生在约 20 亿到 10 亿年前，多个独立记录聚集在约 19 小时；其间亦夹杂日长更快变长的时段，暗示出现潮汐共振使不同扭矩近乎相互抵消。

氧气演化也可能受日长影响：Judith Klatt 等的实验与模型显示，当日长短于约 16 小时时，微生物垫可能净消耗氧气；日长更长则让更多氧气外逸到周遭水体。若日长在约 19 小时附近「卡住」达 10 亿年，或可解释该时段全球氧气长期维持在较低水平，直到日长再次朝 24 小时增长。现代尺度上，原子钟显示日长可在年际间漂移数千分之一秒；1962–2012 的分析在去除大气与海洋影响后，仍可用两个主要特征描述一年以上的变化，包含 1–10 年变化、约 5.9 年振荡，以及与「地磁急变」同时出现的跳变，指向外核流动可使日长以毫秒的分数幅度伸缩。

Research suggests that billions of years ago Earth’s day was not 24 hours; for roughly 1 billion years it may have stayed near 19 hours due to a balance among ocean tides, atmospheric tides, and the Moon’s pull. Normally lunar tidal friction slowly drains rotational energy so day length creeps upward; a NASA overview estimates this lengthens the day by about 0.002 seconds per century.

Ross Mitchell and colleagues compiled dozens of rock-based ancient day-length estimates spanning ~2.5 billion years, using cyclostratigraphic patterns that encode orbital and spin cycles. The record is not a smooth slowdown: it shows long plateaus where day length barely changes. The standout plateau occurs ~2 to 1 billion years ago, when many independent records cluster around ~19 hours, separated by intervals of faster lengthening consistent with tidal resonance nearly canceling opposing torques.

Oxygen dynamics may track these timings: lab/model work by Judith Klatt et al. finds that with days shorter than ~16 hours, microbial mats can be net oxygen sinks, while longer days allow increasing oxygen leakage. A ~19-hour lock lasting ~1 billion years could help explain modest global oxygen through much of that interval until day length resumed growth toward 24 hours. Today, atomic clocks show year-to-year day-length drift of a few thousandths of a second; a 1962–2012 analysis highlights decade-scale modes plus 1–10 year changes, including a ~5.9-year oscillation and jumps aligned with geomagnetic jerks, implying core flow alters day length by fractions of a millisecond.