The difference between the dates in years

[Lance Bachmeier]

On Saturday, 10 February 2024 at 15:53:09 UTC, Alexander Zhirov 
wrote:
> Is it possible to calculate the difference between dates in 
> years using regular means? Something like that
>
>
> ```
> writeln(Date(1999, 3, 1).diffMonths(Date(1999, 1, 1)));
> ```
>
> At the same time, keep in mind that the month and day matter, 
> because the difference between the year, taking into account 
> the month that has not come, will be less.
>
> My abilities are not yet enough to figure it out more elegantly.

I'm assuming you mean you want the number of full years between 
the dates. If so, I use something like this:

```
import std;

void main() {
	writeln(fullYears(Date(1999, 3, 1), Date(1999, 2, 1)));
	writeln(fullYears(Date(2000, 3, 1), Date(1999, 2, 1)));
	writeln(fullYears(Date(2000, 3, 1), Date(1999, 4, 1)));
	writeln(fullYears(Date(2006, 4, 1), Date(1999, 4, 1)));
}

bool earlierInYear(Date date1, Date date2) {
	return date1 < Date(date1.year, date2.month, date2.day);
}

long fullYears(Date date1, Date date2) {
	assert(date1 >= date2, "The first date has to be later");
	if (date1.earlierInYear(date2)) {
		return max(date1.year - date2.year, 0);
	} else {
		return date1.year - date2.year;
	}
}
```