DateTime.ParseExact doesn't work for "yyyy-MM-dd HH:mm:ss"

I have System.DateTime.Now stored in a variable called StartDate (variable is of type DateTime?).

I am converting it to string as below:

string temp = StartDate.Value.ToString("yyyy-MM-dd HH:mm:ss")

Now the temp has value in "yyyy-MM-dd HH:mm:ss".

Now I want to again convert string temp to DateTime so, I am doing as below:

DateTime.ParseExact(temp,"yyyy-MM-dd HH:mm:ss",null)

but this doesn't work.

It always returns date in a same format as System.DatTime.Now. It should return value in "yyyy-MM-dd HH:mm:ss" format.