Quote:
Some of my results are correct, others are occurring a day late on Fridays.
May be your evaluation of correctness of result is wrong too.
Quote:
StartDateMonth {4/25/2019 12:00:00 AM} System.DateTime ----->Correct Date and Day
EndMonth {5/23/2019 12:00:00 AM} System.DateTime
Last thursday of May is 30, not 23 !
Check this part
var e = _dtNew.AddMonths(+1).AddDays(-2);
Months have different number of days!
Your code do not behave the way you expect, or you don't understand why !
There is an almost universal solution: Run your code on debugger step by step, inspect variables.
The debugger is here to show you what your code is doing and your task is to compare with what it should do.
There is no magic in the debugger, it don't know what your code is supposed to do, it don't find bugs, it just help you to by showing you what is going on. When the code don't do what is expected, you are close to a bug.
To see what your code is doing: Just set a breakpoint and see your code performing, the debugger allow you to execute lines 1 by 1 and to inspect variables as it execute.
Debugger - Wikipedia, the free encyclopedia[
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Mastering Debugging in Visual Studio 2010 - A Beginner's Guide[
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Basic Debugging with Visual Studio 2010 - YouTube[
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Debugging C# Code in Visual Studio - YouTube[
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The debugger is here to only show you what your code is doing and your task is to compare with what it should do.