Ideally, Icinga 2 should behave consistenly across platforms. These special
cases only existed because mktime() on Windows behaved differently than the
implementation in glibc. With the introduction of Utility::NormalizeTm(),
there's now consistent behavior and the other expected results for windows are
no longer necessary (ideally, they shouldn't have existed in the first place).
for (const T& needle : haystack) creates the illusion that haystack is a
container of T and we're just borrowing needle. In these cases that's not true.
This allows the function to be used both with a double timestamp or a pointer
to a tm struct. With this, a similar implementation inside the tests can simply
use our regular function.
A day specification like "monday -1" refers to the last Monday of the month.
However, there was an off by one if the first day of the next month is the same
day of the week, i.e. a Monday in this example.
LegacyTimePeriod::FindNthWeekday() picks a day to start the search for the day
in question. When given a negative n to search for the n-th last day, it
wrongly used the first day of the following month as the start and counted it
as if it was within the current month. This resulted in a 1/7 chance that the
result was one week too late.
This is fixed by using the last day of the current month instead.
