Open Radar

 
Summary:
NSCalendar's dateByAddingComponents:toDate:options: method returns a date that is a whole number of milliseconds since the Reference Date, regardless of any fraction of milliseconds present in the original date.

Steps to Reproduce:
1. Create a date whose number of seconds since the Reference Date includes a fraction of a millisecond (i.e., the fractional part of the interval contains non-zero digits after 0.001).
2. Create a Gregorian calendar.
3. Create an NSDateComponents instance.
4. Set the day component to -1.
5. Ask the calendar to add the components to the original date (thereby subtracting one day).
6. Use timeIntervalSinceDate: to subtract the original date from the computed date.

Expected Results:
Since the components in NSDateComponents are all of integral types, the calendar returns a computed date whose interval from the Reference Date has the same fraction of milliseconds as the original date (within the limits of floating-point arithmetic). As such, the interval between the two dates is exactly (or as close to exactly as possible) 86400.0000.

Actual Results:
NSCalendar truncates the computed date to a whole number of milliseconds. As such, the interval between the two dates is almost always greater than 86400.0000.

Regression:
Unknown.

Notes:
I'm including a test app. Here's its output on my system:

Starting date: 258879920.079869
One day earlier: 258793520.079000
Interval between: 86400.000869