Subtraction is simpler after normalization
When two dates are in the same continuous day-number system, the elapsed-day calculation is direct: subtract the earlier day number from the later day number. Month lengths, leap years, and calendar labels have already been resolved.
This is especially useful when two source dates come from different calendar systems. You can normalize both to day numbers, compare the interval, and then keep the original calendar labels for human-readable context.
For example, a historical note can preserve one date as Julian calendar and another as Gregorian, but still compare elapsed days by using the same JDN basis. The same bridge is explained in How to Convert Julian Calendar Dates.
Use the same day boundary for both values
Date differences are only reproducible when both values use the same boundary convention. Civil web tools usually treat dates as midnight-to-midnight calendar days, while astronomical Julian Dates can include fractional days.
If time of day matters, record whether you used an integer Julian Day Number, a fractional Julian Date, UTC, or another time scale. That note prevents small but important offsets.
Keep the day number as audit data
For research notes, software tests, or spreadsheet checks, the intermediate day number is useful audit data. It lets another person reproduce the date difference without reverse-engineering every calendar rule from the final dates.
The final presentation can still use familiar calendar labels. The day-number layer is there to make the calculation transparent.