https://learn.microsoft.com/en-gb/dotnet/api/system.globalization.gregoriancalendar.getdayofyear?view=netframework-4.8.1&viewFallbackFrom=net-9.0
GregorianCalendar.GetDayOfYear(DateTime) Method (System.Globalization) | Microsoft Learn
Returns the day of the year in the specified DateTime.
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