Ingest Pipeline parsing the wrong week


I am trying to parse weekly indices by utilizing the ingest pipelines in elasticsearch but I keep getting a week before the actual week of the year. Would you know what the issue is?

"date_index_name" : {
"date_rounding" : "w",
"index_name_prefix" : "{{fields.index_prefix}}",
"index_name_format" : "YYYY.ww",
"ignore_failure" : false,
"field" : "timestamp"

Today is the 41st week of the year but the my index shows the 40th.

Thanks a lot for your help.