> Plus/Increment/Add 1 month, does have one single meaning
No, it doesn't.
You walk in to the doctor's office on February 28. At the desk, you see another patient about to leave. They turn to the desk attendant and say, "I'll see you in a month for my follow-up."
What date is the other patient's next appointment? What if the date you walked in had been January 31?
Also, for what it's worth, in C#:
DateTime x = new DateTime(2021, 1, 31);
x.AddMonths(1); // Feb 28
x.AddMonths(2); // March 31
x.AddMonths(1).AddMonths(1); // March 28
No, it doesn't.
You walk in to the doctor's office on February 28. At the desk, you see another patient about to leave. They turn to the desk attendant and say, "I'll see you in a month for my follow-up."
What date is the other patient's next appointment? What if the date you walked in had been January 31?
Also, for what it's worth, in C#: