How does one use NSDateFormatter's isLenient option?

Question

I can\'t seem to find any info on this flag, on StackOverflow or elsewhere on the web. Apple\'s own documentation only says:

If a formatter is set to be

Answer 1

Here is an example where the lenient option makes a difference:

let df = DateFormatter()
df.timeZone = TimeZone(secondsFromGMT: 0)
df.isLenient = true
df.dateFormat = "yyyy-MM-dd"
print(df.date(from: "2015-02-29")) // Optional(2015-03-01 00:00:00 +0000)

2015 is not a leap year, so there is no February 29. With isLenient = true, the date is interpreted as March 1. With isLenient = false it is rejected:

let df = DateFormatter()
df.timeZone = TimeZone(secondsFromGMT: 0)
df.isLenient = false
df.dateFormat = "yyyy-MM-dd"
print(df.date(from: "2015-02-29")) // nil