DateCalculatorViewModel - Windows Calculator

Overview

The DateCalculatorViewModel class manages date calculations, including calculating the difference between two dates and adding/subtracting days, months, and years from a starting date. Namespace: CalculatorApp.ViewModel Implements: Windows::UI::Xaml::Data::INotifyPropertyChanged Declared in: DateCalculatorViewModel.h:15

Observable Properties

Mode Properties

IsDateDiffMode

OBSERVABLE_PROPERTY_RW(bool, IsDateDiffMode);

Indicates whether the calculator is in “Date Difference” mode. When true, calculates the difference between two dates.

IsAddMode

OBSERVABLE_PROPERTY_RW(bool, IsAddMode);

Indicates whether the calculator is in “Add/Subtract” mode. When true, adds or subtracts an offset from a start date.

Input Properties (Date Difference Mode)

FromDate

property Windows::Foundation::DateTime FromDate
{
    Windows::Foundation::DateTime get();
    void set(Windows::Foundation::DateTime value);
}

The starting date for date difference calculations.

ToDate

property Windows::Foundation::DateTime ToDate
{
    Windows::Foundation::DateTime get();
    void set(Windows::Foundation::DateTime value);
}

The ending date for date difference calculations.

Input Properties (Add/Subtract Mode)

StartDate

property Windows::Foundation::DateTime StartDate
{
    Windows::Foundation::DateTime get();
    void set(Windows::Foundation::DateTime value);
}

The starting date for add/subtract calculations.

DaysOffset

OBSERVABLE_PROPERTY_RW(int, DaysOffset);

Number of days to add or subtract. Maximum value: 999.

MonthsOffset

OBSERVABLE_PROPERTY_RW(int, MonthsOffset);

Number of months to add or subtract. Maximum value: 999.

YearsOffset

OBSERVABLE_PROPERTY_RW(int, YearsOffset);

Number of years to add or subtract. Maximum value: 999.

OffsetValues

property Windows::Foundation::Collections::IVector<Platform::String^>^ OffsetValues
{
    Windows::Foundation::Collections::IVector<Platform::String^>^ get();
}

Read-only collection of valid offset values (0-999) for ComboBox binding. Constant: c_maxOffsetValue = 999

Output Properties (Date Difference Mode)

StrDateDiffResult

OBSERVABLE_PROPERTY_R(Platform::String^, StrDateDiffResult);

Formatted string showing the difference in years, months, and days. Example: “2 years, 3 months, 15 days”

StrDateDiffResultInDays

OBSERVABLE_PROPERTY_R(Platform::String^, StrDateDiffResultInDays);

Formatted string showing the total difference in days only. Example: “835 days”

IsDiffInDays

OBSERVABLE_PROPERTY_R(bool, IsDiffInDays);

Indicates whether the difference should only be shown in days (when dates are the same or difference is less than 1 month).

StrDateDiffResultAutomationName

OBSERVABLE_PROPERTY_R(Platform::String^, StrDateDiffResultAutomationName);

Accessibility-friendly text for the date difference result.

Output Properties (Add/Subtract Mode)

StrDateResult

OBSERVABLE_PROPERTY_R(Platform::String^, StrDateResult);

Formatted string showing the calculated result date. Example: “Wednesday, March 15, 2023”

StrDateResultAutomationName

OBSERVABLE_PROPERTY_R(Platform::String^, StrDateResultAutomationName);

Accessibility-friendly text for the result date.

Commands

CopyCommand

COMMAND_FOR_METHOD(CopyCommand, DateCalculatorViewModel::OnCopyCommand);

Copies the calculation result to the clipboard. Implementation:

void OnCopyCommand(Platform::Object^ parameter);

Key Methods

Constructor

DateCalculatorViewModel();

Initializes the date calculator with default values and sets up date formatting.

Private Methods

OnPropertyChanged

void OnPropertyChanged(_In_ Platform::String^ prop);

Callback invoked when any property changes. Triggers input validation and result recalculation.

OnInputsChanged

void OnInputsChanged();

Recalculates results when input properties change.

UpdateDisplayResult

void UpdateDisplayResult();

Updates the formatted result strings based on current calculation results.

GetDateDiffString

Platform::String^ GetDateDiffString() const;

Formats the date difference as “X years, Y months, Z days”.

GetDateDiffStringInDays

Platform::String^ GetDateDiffStringInDays() const;

Formats the date difference as total days.

GetLocalizedNumberString

Platform::String^ GetLocalizedNumberString(int value) const;

Converts an integer to a localized string representation.

ClipTime

static Windows::Foundation::DateTime ClipTime(
    Windows::Foundation::DateTime dateTime,
    bool adjustUsingLocalTime = false
);

Removes the time component from a DateTime, keeping only the date.

INotifyPropertyChanged Implementation

The ViewModel uses the OBSERVABLE_OBJECT_CALLBACK macro:

OBSERVABLE_OBJECT_CALLBACK(OnPropertyChanged);

This generates automatic property change notifications and calls the OnPropertyChanged method for custom logic.

Property Macro Usage

Observable Property Example

OBSERVABLE_PROPERTY_RW(bool, IsDateDiffMode);

Expands to:

property bool IsDateDiffMode
{
    bool get() { return m_IsDateDiffMode; }
    void set(bool value)
    {
        if (m_IsDateDiffMode != value)
        {
            m_IsDateDiffMode = value;
            RaisePropertyChanged(L"IsDateDiffMode");
        }
    }
}
private:
    bool m_IsDateDiffMode;
public:

Custom Property Example

The FromDate, ToDate, and StartDate properties are implemented manually to call RaisePropertyChanged:

property Windows::Foundation::DateTime FromDate
{
    Windows::Foundation::DateTime get()
    {
        return m_fromDate;
    }
    void set(Windows::Foundation::DateTime value)
    {
        if (m_fromDate.UniversalTime != value.UniversalTime)
        {
            m_fromDate = value;
            RaisePropertyChanged("FromDate");
        }
    }
}

Usage Examples

Date Difference Mode

// Create ViewModel
auto dateVM = ref new DateCalculatorViewModel();

// Set to Date Difference mode
dateVM->IsDateDiffMode = true;

// Set dates
Windows::Foundation::DateTime from, to;
// ... initialize dates ...
dateVM->FromDate = from;
dateVM->ToDate = to;

// Get results
Platform::String^ difference = dateVM->StrDateDiffResult;
// e.g., "2 years, 3 months, 15 days"

Platform::String^ totalDays = dateVM->StrDateDiffResultInDays;
// e.g., "835 days"

// Copy result
dateVM->CopyCommand->Execute(nullptr);

Add/Subtract Mode

// Create ViewModel
auto dateVM = ref new DateCalculatorViewModel();

// Set to Add/Subtract mode
dateVM->IsAddMode = true;

// Set start date
Windows::Foundation::DateTime startDate;
// ... initialize date ...
dateVM->StartDate = startDate;

// Add 2 years, 6 months, 10 days
dateVM->YearsOffset = 2;
dateVM->MonthsOffset = 6;
dateVM->DaysOffset = 10;

// Get result
Platform::String^ resultDate = dateVM->StrDateResult;
// e.g., "Wednesday, September 25, 2025"

// Copy result
dateVM->CopyCommand->Execute(nullptr);

Binding to XAML

<DatePicker Date="{x:Bind ViewModel.FromDate, Mode=TwoWay}" />
<DatePicker Date="{x:Bind ViewModel.ToDate, Mode=TwoWay}" />

<TextBlock Text="{x:Bind ViewModel.StrDateDiffResult, Mode=OneWay}" />
<TextBlock Text="{x:Bind ViewModel.StrDateDiffResultInDays, Mode=OneWay}" />

<ComboBox ItemsSource="{x:Bind ViewModel.OffsetValues}"
          SelectedItem="{x:Bind ViewModel.DaysOffset, Mode=TwoWay}" />

Accessibility

The ViewModel provides separate automation names for screen readers:

StrDateDiffResultAutomationName - Narrator-friendly date difference
StrDateResultAutomationName - Narrator-friendly result date

These properties format dates and numbers in a way that screen readers can pronounce clearly.

Localization

The ViewModel uses the current system locale for:

Date formatting (DateTimeFormatter)
Number formatting (GetLocalizedNumberString)
List separators in results

All displayed dates and numbers respect the user’s regional settings.

Core Components

ViewModels

Unit Conversion

Graphing

​Overview

​Observable Properties

​Mode Properties

​IsDateDiffMode

​IsAddMode

​Input Properties (Date Difference Mode)

​FromDate

​ToDate

​Input Properties (Add/Subtract Mode)

​StartDate

​DaysOffset

​MonthsOffset

​YearsOffset

​OffsetValues

​Output Properties (Date Difference Mode)

​StrDateDiffResult

​StrDateDiffResultInDays

​IsDiffInDays

​StrDateDiffResultAutomationName

​Output Properties (Add/Subtract Mode)

​StrDateResult

​StrDateResultAutomationName

​Commands

​CopyCommand

​Key Methods

​Constructor

​Private Methods

​OnPropertyChanged

​OnInputsChanged

​UpdateDisplayResult

​GetDateDiffString

​GetDateDiffStringInDays

​GetLocalizedNumberString

​ClipTime

​INotifyPropertyChanged Implementation

​Property Macro Usage

​Observable Property Example

​Custom Property Example

​Usage Examples

​Date Difference Mode

​Add/Subtract Mode

​Binding to XAML

​Accessibility

​Localization

​See Also

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