Precisely. A year is *actually* 365.24 days (give or take). And a day is *actually* as long as it is. And a moon-cycle is *actually* as long as it is, allthough that has little practical significance. (and doesn't match the "month" anyway)
But sure, we could use year day.fraction and do away with months, weeks, hours, minutes and seconds.
2011 150.5 would be mid-day on the 150th day of year 2011.