[zokier]

While deciday is workable hour replacement and milliday is decent minute replacement, the metric time would be missing decent seconds replacement. The next named prefix, microday, is 86ms which is very small for humans.

Of course it is not insurmountable problem, but for lot of other units milli- tends to represent the lower end of "human scale"; millimeters, milligrams, milliliters, milliwatts, etc are as small as most people will ever use in day-to-day life.

In some ways it would be more convenient if the base unit would be e.g. 1e-4 days (or some other power of 10) so day would be 10 kilos. Understandably it would be lot less elegant but more practical. Deci- would be .86 seconds which is kinda nice.

[madcaptenor]

I think what likely happens here is that everyone just starts calling 10^(-5) days (= 0.864 current seconds) a "second". (And then there's confusion over which second people mean, analogous to what happens now with tons.)