If then statement inside a page field? Business central AL - dynamics-business-central

Question

Im trying to create page for giving my to customers discounts on my products, but for a short period of time. I have created a field "start date" and "end date" of this promotion. Next thing i want to do, is to validate the input of the date.
By "Validate" i mean, that start date cannot be greater then end date. I decided to try preventing from writing into "end date" field unless there is a value in "Start date" field, but i ran into some syntax errors... Can you help me with that? Here is the logic i want to write for my page:
field("Starting Date"; Rec."Starting Date")
{
ApplicationArea = All;
}
field("End Date"; Rec."End Date")
{
ApplicationArea = All;
if Rec."Starting Date" = '' then
Editable = false;
}
Here is the full page code i have so far for better understanding:
page 95012 "ArKe Provision Subform"
{
Caption = 'ArKe Provision Subform';
PageType = ListPart;
ApplicationArea = All;
UsageCategory = Administration;
SourceTable = ArKeProvisionLine;
SourceTableView = sorting(Status, "Line No.") order(descending);
layout
{
area(Content)
{
repeater(ProvisionLineRepeater)
{
field(Status; Rec.Status)
{
ApplicationArea = All;
trigger OnValidate()
begin
CurrPage.Update();
end;
}
field("Customer Type"; Rec."Customer Type")
{
ApplicationArea = All;
}
field("Product Type"; Rec."Product Type")
{
ApplicationArea = All;
}
field("Starting Date"; Rec."Starting Date")
{
ApplicationArea = All;
}
field("End Date"; Rec."End Date")
{
ApplicationArea = All;
if Rec."Starting Date" = '' then begin
Editable = false;
end
}
field("Provision %"; Rec."Provision %")
{
ApplicationArea = All;
}
field("Line No."; Rec."Line No.")
{
ApplicationArea = All;
Editable = false;
}
}
}
}
}

Answer 1

You do not have to format "End Date" or "Starting Date", you can just compare them to 0D (or 0DT if their DataType is DateTime)
Like This:
field("Starting Date"; Rec."Starting Date")
{
ApplicationArea = All;
trigger OnValidate()
begin
if Rec."End Date" <> 0D then begin//0DT in case field is DateTime
if rec."End Date" < rec."Starting Date" then begin
Error('Starting date greater then end date!');
end;
end;
end;
}
field("End Date"; Rec."End Date")
{
ApplicationArea = All;
trigger OnValidate()
begin
if Rec."Starting Date" <> 0D then begin//0DT in case field is DateTime
if rec."Starting Date" > rec."End Date" then begin
Error('Starting date greater then end date!');
end;
end;
end;
}

Answer 2

I found the solution! You should convert your date into a string by writing Format(Rec. ...) and then compare it by using <> instead of :=.
code:
field("Starting Date"; Rec."Starting Date")
{
ApplicationArea = All;
trigger OnValidate()
begin
if Format(Rec."End Date") <> '' then begin
if rec."End Date" < rec."Starting Date" then begin
Error('Starting date greater then end date!');
end;
end;
end;
}
field("End Date"; Rec."End Date")
{
ApplicationArea = All;
trigger OnValidate()
begin
if Format(Rec."Starting Date") <> '' then begin
if rec."Starting Date" > rec."End Date" then begin
Error('Starting date greater then end date!');
end;
end;
end;
}