According to the following link from the Smithsonian, July 10th was 1.36ms  shorter, July 22nd 1.34ms and August 5th (predicted) 1.5ms shorter than the standard 24 hours. The explanation is that these shorter day lengths are happening because the moon will be at its farthest angles from the Equator on those days, which weakens the impact of its gravity on Earth. A small amount, but enough to upset high-res GPS.

https://www.smithsonianmag.com/smart-news/today-will-likely-be-shorter-than-usual-and-it-will-happen-again-in-august-heres-why-180986989/ 

The National Geographic comes up with different figures and a different explanation: July 9th [sic] was 1.34ms shorter than the regulation 24 hours, and joins an unusual number of other short days this year, July 22nd and (predicted) Aug 5th.

Studies suggest that factors like melting ice, rising seas, and depleted groundwater are changing Earth’s mass and slowing its spin. This slows its rotation in the same way an ice skater lowers her arms from overhead to stall her spin. ... Inside the planet, something weird is also happening that physicists can’t totally explain. For some reason, Earth’s core has been slowing down for the last 50 years, and to maintain angular momentum, the solid Earth above has been spinning a bit faster to compensate. “We don't know why, or what the core might do in the future,”

https://www.nationalgeographic.com/science/article/shortest-day-earth-rotation 

(asks for email address but keep ignoring the request and it'll let you in)

I prefer the Smithsonian explanation as it has predictability and intervals are close to half a lunar month.

Originally Posted by: DEW