Converting any string to date and considering "no date"

Converting strings to dates never seemed to be straightforward. First, specifying the source for all the different date component was difficult. Second, if the conversion is unsuccessful and "no date" has been defined, it could not be expressed with TDate or TDateTime.

Possible solutions for both issues will be presented in this article.

Format specifiers

You define the source of date components using format specifiers. Think of the like placeholders in your string. Delphi initializes the format settings depending on the system your application runs on. If you run on a US-based system, the short date format string will be initialized with mm/dd/yy. In Germany, it will be dd.mm.yy.

Example

Let us assume we have a string with the following numbers 230623. This is supposed to be interpret as a date. Each part of the consists of two digits:

• Year
• Month
• Day

In order to convert this safely, we can use the following code snippet:

var
  LString: String;
  LPaidOn: TDateTime;
  LFormat: TFormatSettings;
 
begin
  // initialize format
  LFormat.ShortDateFormat := 'yymmdd';
 
  // try to convert...
  if TryStrToDate( LString, LPaidOn, LFormat ) then
  begin
    Result.PaidOn := LPaidOn;
  end
  else
  begin
    // what should we assign?
  end;
 

TryStrToDate will return true if the conversion was successful. The date will be stored in LPaidOn. Otherwise, LPaidOn will remain unchanged.

There is one big issue. How can we assign a value to our variable if the conversion was not successful and we want to indicate that the variable "has no value"? A lot of implementations use a dedicated date that they define as "No value" (for example, a date far in the past or future). However, there is a far better solution!

Introducing Nullable data types

In database programming, we can assign "no value" to a field. Sadly, Object Pascal so far has no "official" support for this in the language. However, 3rd party libraries come to the rescue.

In this case, I use the TMS Business Core Library (BCL).

Import Bcl.Types.Nullable.pas:

uses
  System.Generics.Collections
  , Bcl.Types.Nullable

Declare your date time value as Nullable. This can also be a field variable of your class. It is not restricted to local variables, for example.

FPaidOn: Nullable<TDateTime>;

Assign values as before or set it to "no value". This is done in the BCL using the variable SNull.

  if TryStrToDate( LString, LPaidOn, LFormat ) then
  begin
    FPaidOn := LPaidOn;
  end
  else
  begin
    FPaidOn := SNull;
  end;

In order to query if a value is assigned or not, you can use IsNull.

  if FPaidOn.IsNull then 
  begin
    // value is null
  end
  else
  begin
    // use value
  end;

You might be hesitant to use this fearing it might break IDE functionality like Code Completion. However, it works perfectly fine as at its core the Nullable implementation uses nothing that would confuse the IDE.

List of format specifiers

I always struggle to remember which specifiers can be used with TFormatSettings. Here is the table based on the Embarcadero documentation at https://docwiki.embarcadero.com/Libraries/Alexandria/en/System.SysUtils.FormatDateTime.