What abjad numerals are
The abjad system assigns a numeric value to each Arabic letter, an ordering that predates the modern alphabetical sequence and was used for numbering long before Arabic numerals were standard. Converting a name through abjad is arithmetic, and these tools show the working.
Scholarly opinion on using such values for guidance varies widely, and we do not take a position on it. What we can do is show the calculation clearly and say where it comes from.
Dates and the Hijri calendar
The Islamic calendar is lunar, so a Hijri date drifts against the Gregorian one by roughly eleven days a year. Conversion tools here follow the standard tabular calculation, which can differ by a day from local moon sighting.
For anything that matters religiously, follow your local authority rather than a calculator.