There are different conventions to determine the first week of the year. Sunrise wants to be in line with European bookkeepers and they are using the ISO convention.
Long story short: week 1 is the first week of the year containing a Thursday.
In excel the right function to do this would be: ISOWEEKNUM()
Links:
- https://www.iso.org/iso-8601-date-and-time-format.html?utm_source=syndication
- https://www.yahoo.com/tech/weeknum-vs-isoweeknum-whats-difference-200014930.html?guccounter=1&guce_referrer=aHR0cHM6Ly93d3cuZ29vZ2xlLmNvbS8&guce_referrer_sig=AQAAAGU4LVpjSE7QBYhT81TX5Ti_dPJsRb4fW2JmxUjN0hdnowX2CshVykt2U0Vr5vNeDkAA3H8ghsT_82E7inln5Hl0bfnJ6GGAZ44shVAfv3e0nlGQ1yoKldIB3rvyO7hiGho1fqQY1ZFs0Z6z-1KI_LtXAqD8VXSuVdq_WZrBgtGd