Two calendars name the same days differently, which sounds like a conversion problem and mostly is not. The question that causes trouble is what “the difference between these dates” means — because one reading of it survives the change of calendar untouched and the other does not.
How to count the days between two Hijri dates
Convert both, then subtract
A day is a day in every system, so the distance between two moments does not depend on what they are called. Convert each endpoint once and the day count is exact. Months and years are the ones that have to be counted in the calendar you were asked in.
What actually changes
A calendar is a naming scheme laid over a continuous run of days. Change the scheme and every name changes; the days underneath do not move.
So a gap measured in days is invariant. If two events are 214 days apart, they are 214 days apart whether you write the endpoints as Hijri or Gregorian, and a converter that disagrees has a bug. This is the useful half of the subject and it is rarely stated.
A gap measured in months or years is not invariant, because the units themselves differ. A Gregorian month is about 30.44 days on average; a Hijri month is 29.53. Three months is a different quantity of time in each, so “three months from now” lands on two different days depending on which calendar the sentence was written in — and neither is wrong.
Why the calendars drift
The Hijri year is twelve lunar months and nothing else. A lunar month averages 29.53059 days, so a Hijri year runs about 354.367 days against the Gregorian 365.2425 — roughly eleven days shorter, every year, with no correction.
The Gregorian calendar inserts a day almost every fourth year specifically to stay locked to the seasons. The Hijri calendar has no such mechanism because it is not trying to: it tracks the moon, and the seasons are allowed to move. Ramadan arriving eleven days earlier each year is the system working.
Eleven days a year accumulates. After about thirty-three years the shortfall reaches a whole year, and the two counts differ by one — thirty-three Hijri years are close to thirty-two Gregorian ones. That is also the answer to a question people ask about themselves: your age in Hijri years is larger than your age in Gregorian years, by one for every thirty-three you have lived.
The month that is not decided yet
This is the part that has no equivalent on the Gregorian side and that catches people out.
A Hijri month is 29 or 30 days long depending on when the new crescent first becomes visible. That is an observation, not a formula, so the length of next month is not known with certainty until it begins. Different authorities resolve it differently: Saudi Arabia publishes the Umm al-Qura calendar, computed astronomically in advance, while other countries announce the new month after a sighting.
The consequence is that two correct Hijri calendars can name the same day differently, usually by one day. It is not an error in either, and it is why a converter’s output is best treated as “the Umm al-Qura reading” rather than as the date. Where a document gives both a Hijri and a Gregorian date, do the arithmetic on the Gregorian one — it names the same day and it removes the ambiguity.
What the tools here do, and what they do not
Said plainly, because the limitation matters more than the feature.
The date tools on this site are Gregorian. There is no Hijri conversion here, and the date difference calculator will not read a Hijri date if you type one.
What that still leaves is most of the practical work. Convert your two dates to Gregorian in whichever converter your authority publishes, and every day-counted question is answerable here: how many days until an expiry, how many days a period ran, how many days remain of a notice. The age calculator is Gregorian in the same way — it will tell you an age in Gregorian years, and the Hijri figure is that number plus roughly one for every thirty-three.
What it will not do is add three Hijri months to a Hijri date. That genuinely needs a Hijri implementation, and pretending otherwise by adding 88 days would be wrong in a way that looks right.
The traps worth knowing
A visa or a residence permit expiring on a Hijri date. Count the days, not the months. A permit valid “for one year” from a Hijri date expires about eleven days earlier in Gregorian terms than the same phrase would suggest, and people miss renewals on exactly this.
A birth year converted the lazy way. Dividing or multiplying by 33 to move a year between calendars gives an approximate year and never a date. The month and the day matter, because the offset between the two calendars changes continuously through the year rather than annually.
Two documents, one day apart. Before treating it as a discrepancy, check whether the two issuers use computation and sighting respectively. It is the commonest reason for a one-day difference and it is not a mistake by anybody.
And the trap that is not specific to either calendar at all: whether the first and last day are both counted changes the answer by one, in every calendar, and causes more disputes than the eleven-day drift ever has.
The short version
Days are calendar-independent, so convert the endpoints once and count in days whenever the question allows it. Months and years are not, so those have to be counted in the calendar they were asked in. The Hijri year is about eleven days shorter, the two counts separate by a whole year every thirty-three, and a one-day disagreement between two authorities is ordinary rather than an error.