They match up with ISO week numbers, where week 1 always starts on a Monday.
I regularly see ISO weeks used to mark manufacturing dates, where they often offer the right levels of precision, as many processes run on week-long cycles. Often in stampings the year will be replaced by a letter code with an epoch particular to the company.
The ISO week date system is a leap week calendar system that is part of the ISO 8601 date and time standard. The system is used (mainly) in government and business for fiscal years, as well as in timekeeping.
International Standards Organization years; standardizing date-time formats across various UNIX systems. Many people from Europe, for example, prefer the month to be stated after the day of the month. "2 January YYYY" whereas in other places (in formal writings) people prefer the month to be written first: "January 2, YYYY". Sometimes (if, for example, running virtual host) they can appear to be a mix-up since there's quite a bit of ambiguity within the %m%d%H%_M% or "%m%d%H(M)" string sequences.
For sources, see also "errata" on intuitive.com/wicked
when the correct time to use them is?
Basically amounts to personal preference since options can be formatted.
