Steps to Reproduce

1. load the zul below

<datebox weekOfYear="true" />

Current Result

The 1st Thursday is not in the 1st week.

Expected Result

verified it with https://www.timeanddate.com/calendar/?year=2012&country=1&wno=1

Debug Info

• if you set the first day of the week to Monday, then the week number is correct. It matches the result from timeanddate.com.
• week number definition: https://www.epochconverter.com/weeknumbers. There are 3 week numbering systems: ISO week date standard (ISO-8601), US, Islamic.
• check 2016/1/1