Weeks between dates
Weeks are days divided by seven. Ninety days is 12.857 weeks, or twelve whole weeks and six days. Both forms are shown, because a decimal week is rarely what anyone actually wants to act on.
How to count weeks between dates
Pregnancy is where week counting matters most and where the convention is least obvious: gestational age is counted in completed weeks plus days, written as "24w 3d", from the last menstrual period rather than from conception. Project planning uses the opposite convention, rounding partial weeks up because a sprint is a sprint. Neither is wrong; they are counting different things. The week is also the only everyday unit that never changes length, and the consequence of that is less obvious than it sounds: 52 weeks is 364 days, a day short of a year, so any weekly cycle drifts against the calendar by one day a year and two across a leap year. A given weekday therefore falls 53 times in roughly one year in six, which is why weekly payroll has a name for the extra pay period and a rule for taxing it, and why a salary divided by 52 does not quite survive contact with the calendar.
Questions
Divide the days by seven. Ninety days is 12 weeks and 6 days.
52.14 in a common year, 52.29 in a leap year. Never exactly 52.
Because 52 weeks is only 364 days. The leftover day means one weekday occurs 53 times, which happens in about one year in six.
It counts completed weeks plus days from the last period, which is a different convention from a plain division.
For scheduling, usually yes. For measurement, report the remainder instead — rounding 12.857 to 13 hides nearly a full day.
No. ISO weeks are positions in the calendar year and always start on a Monday; this counts elapsed weeks from whatever day you gave it.
14 weeks and 2 days.