This term is often used specifically to refer to the French Republican calendar time system used in France from 1794 to 1800, during the French Revolution, which divided the day into 10 decimal hours, each decimal hour into 100 decimal minutes and each decimal minute into 100 decimal seconds (100,000 decimal seconds per day), as opposed to the more familiar standard time, which divides the day into 24 hours, each hour into 60 minutes and each minute into 60 seconds (86,400 SI seconds per day).
Changing the second would make a lot of calculations in the metric systems that are very clean in terms of constants now very ugly though.
There is no relationship between the metric constants and the current rotation rate of the planet. We can change the second to meet the needs of the constants, but time units would no longer align with the days.