When reading the date portion, Calc first checks to see if it is an ISO 8601 week-numbering date; if the string contains an integer representing the year, a “W” followed by two digits for the week number, and an integer from 1 to 7 representing the weekday (in that order), then all other characters are ignored and this information determines the date.
Emacs does it, see https://www.gnu.org/software/emacs/manual/html_mono/calc.html#Date-Formats:
Consider doing the same.
Depends on #53