DateTime.Parse can format unusual format strings?

Question

I was looking at a code in an application (Someone else wrote it),on some cases it worked fine and on some cases it gave exceptions,it was actually converting strings in datetime,here is the code

//5000 is the year,but what about "1" is it month or day ?,if its month
//then what about the day ?
DateTime time =  DateTime.Parse("1.5000");//1.5000 doesn't looks a date to me ?
time.ToString();//returns "1/1/5000 12:00:00 AM"

//where as if I give this string to DateTime.Parse();
time =  DateTime.Parse("2341.70");
//FormatException was unhandled
//String was not recognized as a valid DateTime.

A Confusing thought
How does this string "3.5000" (it matches the 1.5000 pattern) evaluates , does this means 3-3-5000 or 1-3-5000 ,the format is ambiguous its unclear and confusing !

My questions are,

1. What kind of formats can DateTime.Parse expects ?
2. Whats happening in the code above ?
3. Suggestions to improve the code ?

Answer 1

Many people have commented on the possible reasons for the parse that you have seen being successful but your question seems to have several separate parts...

1. What kind of formats can DateTime.Parse expects ?

DateTime.Parse has been written to be as inclusive as possible. Pretty much anything that it can find someway to make into a DateTime it will do its best to do so which means in addition to the usual familiar yyyy-MM-dd type formats more strange ones like M.yyyy or yyyy.M and so on.

2. Whats happening in the code above ?

That is very complicated because the DateTime.Parse method is itself very complicated. You can probably fidn the source code out there somewhere but the complexity made it very hard for me to follow. Without being able to give precise details I'm going to answer this the same as above. What is happening is that the framework is trying its best to give you a date back and not throw an exception. The date it gives is the best guess as to what you meant.

3. Suggestions to improve the code ?

It sounds like if you are getting parse exceptions that you are passing dates in formats that are unexpected. Without knowing what those inputs are its hard to say. Two things could improve your code though. Making sure a single consistent date format is used and then using DateTime.ParseExact to ensure that it conforms to the right format. You will remove all ambiguity this way but you will sacrifice flexibility.

The second option is to use DateTime.TryParse. This will attempt to parse your date and then return a boolean saying whether it succeeded or not. If successful the date parse will be returned in a ref parameter. This won't make your code any better at recognising unknown date formats but will let your code know when such an unparsable format crops up and you can deal with it (eg by providing user feedback reporting the wrong format and suggesting a correct one, or just by logging it or something else).

What the best method is depends mostly on where your input is coming from. If it is user input then I'd go with the second option. If it is automated input then you probably want to make sure your input is standardized and then use the first option. Of course circumstances always vary so this is not a hard and fast rule. :)