[couchbase] iOS date problems

I'm experiencing some strange problems with dates being saved/loaded in the DB as null values. It is very random. So far it is happening for different locales, different time zones. Sometimes in a single object that has 2 fields with dates - one might get it right and the other one get as null. I'm not able to determine any pattern.

Looking at the code seems you are using NSDateFormatter to serialize dates using their JS .toISOString(). Reading through some best practices, seems when you want a fixed format it is best you also set the local to en_US_POSIX to avoid problems:
https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/DataFormatting/Articles/dfDateFormatting10_4.html#//apple_ref/doc/uid/TP40002369-SW7

Also for iOS10+ it is recommended to use the newly introduced NSISO8601DateFormatter
https://developer.apple.com/documentation/foundation/nsdateformatter#2528261
https://developer.apple.com/documentation/foundation/nsiso8601dateformatter

What do you think to tweak the logic and use either ot those?

Also is there any reason why you have made the format use the Z timezone pattern, instead of the char Z. So you have

dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZ"

instead of

dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"

Here is an example of how the string format looks like with either of those: https://swiftfiddle.com/?c=H4sIAAAAAAAAA6tWSlayUsrMLcgvKlFwyy%2FNS0ksyczPiwHCnNQShQoFWwWXxJJUt%2Fyi3MSSktQiDU2QVIVeClwQqCJGqRIIdH19dVNS1EPUPTyscnOtiov1goODo2KUYvIKijLzSjQq9IpLgIx0jbSi%2FFwrsLF6efnlmiSaqB6lToSZSjpKZUCPmekZ6Bkp1QIAEfFcXeYAAAA%3D

Start by tracing the iOS date serialization and deserialization logic that uses NSDateFormatter, focusing on locale, time-zone, and ISO 8601 handling. Reproduce the null-date behavior across the reported locales and time zones; done means dates consistently round-trip instead of becoming null.