C++ Implementation of Excel Date Functions: Basics of Time Handling

Hello everyone! Today, I will guide you through implementing commonly used date handling functions in C++.

1. Basic Knowledge of Excel Date Functions

The commonly used date functions in Excel can be categorized into three types:

1. Extraction Functions: YEAR, MONTH, DAY, etc.
2. Calculation Functions: DATEDIF, EDATE, etc.
3. Conversion Functions: DATEVALUE, TEXT, etc.

To implement these functionalities in C++, we need to master:

• <span><chrono></span> standard library: Modern C++ time handling tools
• <span><ctime></span> traditional library: C-style time functions
• Date algorithms: such as calculating date differences, adding months, etc.

2. Basic Date Structure Design

1. Simple Date Class

#include <iostream>
#include <stdexcept>

class SimpleDate {
private:
    int year, month, day;
    
    bool isValid() const {
        if(year < 1900 || month < 1 || month > 12 || day < 1) 
            return false;
        
        static const int daysInMonth[] = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
        int maxDay = daysInMonth[month-1];
        
        // Handle leap year for February
        if(month == 2 && (year % 400 == 0 || (year % 100 != 0 && year % 4 == 0))) {
            maxDay = 29;
        }
        
        return day <= maxDay;
    }
    
public:
    SimpleDate(int y, int m, int d) : year(y), month(m), day(d) {
        if(!isValid()) {
            throw std::invalid_argument("Invalid date");
        }
    }
    
    int getYear() const { return year; }
    int getMonth() const { return month; }
    int getDay() const { return day; }
    
    // Similar to Excel's YEAR function
    int yearFunc() const { return year; }
    
    // Similar to Excel's MONTH function
    int monthFunc() const { return month; }
    
    // Similar to Excel's DAY function
    int dayFunc() const { return day; }
};

2. Usage Example

int main() {
    SimpleDate today(2023, 5, 15);
    
    std::cout << "YEAR: " << today.yearFunc() << "\n"
              << "MONTH: " << today.monthFunc() << "\n"
              << "DAY: " << today.dayFunc() << std::endl;
    
    return 0;
}

Tips:

• Validate date validity in the constructor
• Leap year determination is a common challenge in date handling
• Support for more formats of date string input can be extended

3. Implementing Common Excel Date Functions

1. DATEDIF Function (Calculate Date Difference)

// Calculate the difference in days between two dates
int dateDiff(const SimpleDate& date1, const SimpleDate& date2) {
    // Convert to time_t for calculation
    std::tm tm1 = {0, 0, 0, date1.getDay(), date1.getMonth()-1, date1.getYear()-1900};
    std::tm tm2 = {0, 0, 0, date2.getDay(), date2.getMonth()-1, date2.getYear()-1900};
    
    time_t time1 = std::mktime(&tm1);
    time_t time2 = std::mktime(&tm2);
    
    if(time1 == -1 || time2 == -1) {
        throw std::runtime_error("Date conversion failed");
    }
    
    return std::difftime(time2, time1) / (60 * 60 * 24);
}

2. EDATE Function (Add Months)

SimpleDate addMonths(const SimpleDate& date, int months) {
    int totalMonths = date.getYear() * 12 + date.getMonth() - 1 + months;
    int newYear = totalMonths / 12;
    int newMonth = totalMonths % 12 + 1;
    int newDay = std::min(date.getDay(), daysInMonth(newYear, newMonth));
    
    return SimpleDate(newYear, newMonth, newDay);
}

3. WEEKDAY Function (Day of the Week)

// Returns 1 (Sunday) to 7 (Saturday), consistent with Excel
int weekDay(const SimpleDate& date) {
    std::tm tm = {0, 0, 0, date.getDay(), date.getMonth()-1, date.getYear()-1900};
    
    if(std::mktime(&tm) == -1) {
        throw std::runtime_error("Date conversion failed");
    }
    
    return tm.tm_wday + 1; // tm_wday is 0-6 (Sunday to Saturday)
}

4. Modern C++ Implementation (C++20’s )

1. Using C++20 Calendar and Time Zone

#include <chrono>
using namespace std::chrono;

year_month_day excelLikeDate(int y, int m, int d) {
    auto ymd = year(y)/month(m)/day(d);
    if(!ymd.ok()) {
        throw std::runtime_error("Invalid date");
    }
    return ymd;
}

// Calculate the difference in days between two dates
int modernDateDiff(const year_month_day& date1, 
                  const year_month_day& date2) {
    auto sys_days1 = sys_days(date1);
    auto sys_days2 = sys_days(date2);
    return (sys_days2 - sys_days1).count();
}

2. Adding Workdays Functionality

// Calculate the number of workdays between two dates (excluding weekends)
int workDaysBetween(const SimpleDate& start, const SimpleDate& end) {
    int days = dateDiff(start, end);
    int weeks = days / 7;
    int remainder = days % 7;
    
    int workDays = weeks * 5;
    
    int startWeekDay = weekDay(start);
    for(int i = 0; i < remainder; ++i) {
        int currentDay = (startWeekDay + i) % 7;
        if(currentDay != 1 && currentDay != 7) { // Not a weekend
            workDays++;
        }
    }
    
    return workDays;
}

5. Date Formatting Output

1. Similar to Excel’s TEXT Function

#include <sstream>
#include <iomanip>

std::string formatDate(const SimpleDate& date, const std::string& format) {
    std::tm tm = {0, 0, 0, date.getDay(), date.getMonth()-1, date.getYear()-1900};
    std::stringstream ss;
    
    if(format == "YYYY-MM-DD") {
        ss << std::setfill('0') 
           << date.getYear() << "-"
           << std::setw(2) << date.getMonth() << "-"
           << std::setw(2) << date.getDay();
    }
    else if(format == "MM/DD/YYYY") {
        ss << std::setfill('0')
           << std::setw(2) << date.getMonth() << "/"
           << std::setw(2) << date.getDay() << "/"
           << date.getYear();
    }
    // Other formats...
    else {
        char buffer[80];
        strftime(buffer, sizeof(buffer), format.c_str(), &tm);
        ss << buffer;
    }
    
    return ss.str();
}

6. Considerations

1. Time Zone Handling:

• Excel dates typically do not consider time zones
• The C++20 <span><chrono></span> library can handle time zones

• Excel’s date range is larger (after January 1, 1900)
• C++’s <span><ctime></span> typically supports dates after 1970

// Functions that are called frequently can be pre-computed
static std::tm toTM(const SimpleDate& date) {
    return {0, 0, 0, date.getDay(), date.getMonth()-1, date.getYear()-1900};
}

7. Hands-On Practice

Try these interesting exercises:

1. Implement an Excel NETWORKDAYS function (calculate workdays between two dates)
2. Add support for dates before January 1, 1900
3. Create a date parser that supports multiple string formats
4. Implement Excel-like date sequence generation functionality

Conclusion

Today, we delved into:

• Design and validation of a basic date class
• Implementation of commonly used Excel date functions
• Usage of the modern C++ <span><chrono></span> library
• Date formatting and workday calculations

Date handling is a common requirement in practical applications, and mastering these techniques can make your programs more professional. It is recommended to start with simple date extraction and gradually implement more complex date calculations.