"How long ago was that?" sounds like a single question, but it actually has two different valid answers depending on whether you mean elapsed time (a fixed, measurable duration, like exactly 45 days) or calendar duration (a calendar-based description, like "a month and a half"). These aren't the same thing, and conflating them is a common source of confusion in date calculations.
Elapsed time is a fixed quantity
Elapsed time measures a duration in fixed, unchanging units, seconds, minutes, hours or days, that don't vary based on which calendar dates they happen to span. Two hours is always exactly two hours, whether it's measured on a Tuesday or spans midnight into Wednesday. This makes elapsed time straightforward to add, compare, and convert between units, since a day is always 24 hours and an hour is always 60 minutes, with no calendar-dependent exceptions. The Time Duration Calculator works in these fixed units, useful for anything measured strictly as elapsed time rather than described relative to a calendar.
Calendar duration is context-dependent
Calendar duration units, month and year specifically, don't have a fixed length in days. "One month from now" can mean anywhere from 28 to 31 days depending on which month it lands in, and "one year from now" means 365 days in a common year but 366 in a leap year. This is why calculating "3 months" as "90 days" is only an approximation, sometimes it's exactly right, sometimes it's off by a day or two, depending on which specific months are involved. The Date Difference Calculator reports duration in calendar terms (years, months, days), correctly handling this variable length rather than assuming a fixed day count per unit.
Why the distinction matters in practice
The distinction matters most when precision is required: a legal deadline described as "90 days" and one described as "3 months" from the same start date can land on genuinely different end dates, since one is a fixed elapsed-time count and the other is a calendar duration that depends on which months it spans. Similarly, an exact age expressed in years and months is a calendar duration, while the same age expressed in total days lived is an elapsed-time figure, both are correct, they're just answering slightly different questions about the same span of time.