2019 calcul de la semaine incorrecte par le masque de format de date standard

参照 ?strftime :

 ‘%U’ Week of the year as decimal number (00-53) using Sunday as
      the first day 1 of the week (and typically with the first
      Sunday of the year as day 1 of week 1).  The US convention.

 ‘%V’ Week of the year as decimal number (01-53) as defined in ISO
      8601.  If the week (starting on Monday) containing 1 January
      has four or more days in the new year, then it is considered
      week 1.  Otherwise, it is the last week of the previous year,
      and the next week is week 1.  (Accepted but ignored on
      input.)

 ‘%W’ Week of the year as decimal number (00-53) using Monday as
      the first day of week (and typically with the first Monday of
      the year as day 1 of week 1).  The UK convention.

Le masque de format IW dans Oracle renvoie le Numéro de semaine ISO de l'année, c'est-à-dire %V en R :

R> x <- as.Date("2018-12-28") + 0:10
R> data.frame(date=x, weeknum=format(x, "%V"))
         date weeknum
1  2018-12-28      52
2  2018-12-29      52
3  2018-12-30      52
4  2018-12-31      01
5  2019-01-01      01
6  2019-01-02      01
7  2019-01-03      01
8  2019-01-04      01
9  2019-01-05      01
10 2019-01-06      01
11 2019-01-07      02