How to get a valid Date as input from GUI - datetime

Question

I have a school practical where users enter their date of birth in a TEdit on a TForm. The user will most likely enter the date according to their custom date settings, but I want to be able to convert any or at least most date formats to the one I have stored in a .mdb Access database. How do I get an accurate and valid date from the GUI as user input?
I know of the decodedate(Date, year, month, day) procedure, but what if the user enters the date differently than system settings or as is displayed in the TDBGrid connected to the .mdb database via an ADOConnection and ADOTable? Then the procedure will crash.

Answer 1

Be a help to the user instead of a pain:
Is "contains" meant literally, so the user can actually also enter dates with leading and/or trailing spaces? Such things occur, especially when copying texts from elsewhere. Trimming off such whitespaces is easy, so allow this. See trim()
Avoid generic error messages but instead make them as precise as possible:
Wrong length? Tell what you expect and what you got instead, i.e.: "Expected length: 10, but got 8 instead."
No delimiter at expected position? Tell so, i.e.: "Expected delimiter "/" as 5th letter, but got "3" instead."
Illegal month? Tell so, i.e: "Expected "1" or "01" thru "12" as month, but got "23" instead."
Try to collect multiple error messages at once, so the user can see all of his mistakes and fix them in one go, instead of being penalized again and again with just one message per take.
Detect patterns and make them restrictive:
Pos #1 is in ['0'.. '2'] and pos #2 thru #4 is in ['0'.. '9']? Looks like a year from which we can proceed:
Pos #5 is in ['/', '-', '.']?
Pos #6 is '0' and pos #7 is in ['1'.. '9']? Or pos #6 is '1' and pos #7 is in ['0'.. '2']?
...and so on. That way you can much better recognize which format has been chosen and you can give much more precise error messages. It will also lead to ambiguous input, such as 4/10/2016 from which you don't know if 4 is a day or a month.
Check against illegal dates: which months cannot have a 31th day? For February 29th see IsLeapYear()
Display the date according to what you recognized - this helps you and your user to make sure the recognition was correct. Ideally you have your three Integers of year, month and date and then convert them to text again, along with delimiters - the outcome should be in general the input you had to deal with.
Regular expressions are great in recognizing patterns and then accessing the found parts. However, it's a topic on its own and should only be used when known on its own. A regex for the format I used in this example would be /([0-2][0-9]{3})[/.-](0?[1-9]|1[0-2])[/.-](0?[1-9]|[12][0-9]|3[01])/ where you can easily access $1 as year, $2 as one or two digit month and $3 as one or two digit day. You "only" have to check against illegal dates.

Answer 2

As #AndreasReijbrand pointed out, a TDateTimePicker would be the correct method to receive a date as user input on a GUI. The TDateTimePicker does all the validation work for you. After much debate, we were eventually allowed to use it. I post this new answer with some important insights to the TDateTimePicker component:
The TDateTimePicker automatically changes to the local set date format of the pc. So validation of that is unnecessary.
Conversion of date display format (yyyy/mm/dd or yyyy-mm-dd) from the .mdb Access database to the TDateTimePicker and vice versa is unnecessary. This is also done automatically. (Starting to see why there was so much criticism for the original lack of use of this component.)
The DateTimePicker does not allow negative values or non-sensical values such as (0030/09/08 or 2003/89/89).