I found a workaround for searches and dashboard is to manually extract them after the search using a strftime
… | eval weeknumber=strftime(_time,"%U") | stats count by weeknumber
To avoid confusions between years, I like to use the year, that help to sort them in chronological order.
… | eval dateyearweek=strftime(_time,"%Y-%U")| stats count by dateyearweek
Depending of your country, you have 2 variations :
%U is replaced by the week number of the year (Sunday as the first day of the week) as a decimal number [00,53].
%V is replaced by the week number of the year (Monday as the first day of the week) as a decimal number [01,53]. If the week 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.