Cocoa Core Data Timestamp Converter. Seconds Since 2001

The 2001 epoch, and why it exists

NeXTSTEP's engineers picked 2001 as the reference date for the NSDate type. A round "start of the millennium" that kept epoch values small and readable in the platform's early years. Today a Core Data Date attribute is stored as seconds since that instant, so 807753600 is 2026, not 1995. Converting is simple: Unix time = cocoa seconds + 978,307,200 (the 1970→2001 gap).

Where you meet Cocoa time

• Core Data SQLite stores, the Z… date columns hold Cocoa seconds; a value like 8.069724e8 is a float in the raw DB.
• iCloud and CloudKit records. Exported NSDate values and sync payloads use the same epoch.
• Plist exports, macOS tools that dump .plist files often show these seconds before any app-layer formatting.
• Swift code. Date().timeIntervalSince1970 returns Unix seconds, but timeIntervalSinceReferenceDate returns Cocoa seconds; mixing them is a classic off-by-31-years bug.

Core Data values, decoded

• 807753600 → Aug 7, 2026 00:00:00 UTC. Check: 807,753,600 + 978,307,200 = 1,786,060,800 seconds = Aug 7, 2026 00:00:00 UTC. (Watch out: 806972400 is a value often quoted for this example, but it decodesto Jul 28, 2026 23:00 UTC, and read as plain Unix time it is Jul 28, 1995 23:00 — mixing the two clocks is exactly the classic Cocoa bug.)
• 0 → Jan 1, 2001 00:00:00 UTC, the epoch itself.
• 807753600.5 → Aug 7, 2026 00:00:00.5 UTC. The tool accepts fractional seconds, exactly as Core Data stores them — but note display precision is whole seconds, so the half second is preserved in the Unix-seconds row, not the clock digits.

NSDate and Cocoa seconds in your own code

• Swift: Date(timeIntervalSinceReferenceDate: cocoaSeconds) to decode; date.timeIntervalSinceReferenceDate to encode.
• Python: datetime.fromtimestamp(cocoa + 978307200, tz=timezone.utc).
• SQL: datetime(cocoa + 978307200, 'unixepoch') in SQLite.

Gotchas: the 31-year shift, fractions, and negative values

• Reference vs Unix, never feed Cocoa seconds into a Unix formatter; the ~31-year offset puts every date in the wrong decade.
• Fractions. Core Data stores sub-second precision as a decimal part. The converter accepts and returns fractional seconds.
• Negative values, anything before 2001 is a negative number. The converter handles them, back to the earliest representable dates.