* math is 0 based while ordinal is 1 based => fix as 1 based logic
* add extensive testing against the "date" command format
* format: test sample instead of every day
* 2007 starts with saturday
* Last day of the year is thus the 52 on Monday weekly calendar, 53 on Sunday weekly calendar.
* update %U expected value in test
* Was the goal was to have a different value than with %W at next line ? another date to pick ?
* update cfg("unix") into cfg(target_os = "linux")
* format tests/dateutils.rs
