How does Swift manage dates in CoreData

Question

I have been including a date in my CoreData, and was curious too see what happens behind the scenes with the underlying SQLite database.

I noticed that while integer and varchar are are used for other CoreData attributes, a CoreData Date attribute appears in SQLite as timestamp.

Now I know that SQLite has its quirks, notably:

• Columns have a type affinity rather than a fixed type, so you can put whatever you like wherever you like
• You can make up your own column types
• There is no distinct date/time type, and that SQLite has conversion functions to work with string, integer or real representations of the date/time

I also notice that the date is stored in a human-readable format. However, it appears to be a non-standard format which is also off by a few years.

For example:

• The SQLite version is 1989/10/05 13:15:45
• The IS8601 formatted version is 2020-10-05T14:15:45+11:00

The ISO8601 exported version above is correct. The SQLite version above is off by 31 years, ignores our DST offset, and doesn’t conform to any standards that I’m aware of.

How does CoreData mange its dates?

Answer 1

See the documentation for Swift Date. Apple use a different reference date (1/1/2001) from Unix (1/1/1970), which explains the bulk, if not all, of the difference.