### 2.11.3. Dates (class date)

Learning objectives

 Date calculations with the class date

In many programs date calculations must be performed. Typical problem definitions are the following:

• Which day of the week falls on a particular date?

• How many days (months, years) are between two particular dates?

• A date is given. Which date is exactly n days (months, years) before or after this date?

• Which day in the year is a particular date?

• Is a particular year a leap year?

To be able to handle dates comfortably and to solve such problems, LEDA offers the class date.

An object of the type date represents a date, consisting of a day d, a month m and a year y. All dates adhering to the Gregorian calendar between January 1st, 1 A.D., and December 31st, 9999 A.D., are permitted.

As an example of the class date we calculate how many days old we are. For this we read day, month and year of our birthday, construct a date from these data, get ourselves a second date which represents the date “today” and calculate the difference (measured in days) of the two dates.

Filename: AgeInDays.C
```#include <LEDA/date.h>
#include <iostream>

using leda::date;
using std::cin;
using std::cout;

int main()
{
cout << "Please enter you birth date:\n";

int b_day, b_month,b_year;

cout << "Day: ";   cin >> b_day;
cout << "Month: "; cin >> b_month;
cout << "Year: ";  cin >> b_year;

date birthdate(b_day, date::month(b_month), b_year);

date today;
today.set_to_current_date();

int age_in_days = today - birthdate;

cout << "You are " << age_in_days << " days old.\n";
}
```

For the top secret data of the author the program spits out the following:

```Please enter your birth date:
Day: 11
Month: 1
Year: 1967
You are 13252 days old.```

Hmmm, some girls call me old...

A date can be constructed from a day, a month and a year. Day and year are of type int. The month is of the enumeration type date::month, which comprises the integer numbers from 1 to 12; these can be specified in the form date::Jan, date::Feb etc.

The method

`D.set_to_current_date();`

sets a date D to the present day (present according to the system time). The default constructor also sets a date to the date of the present day.

If two dates are subtracted by the minus operator, the difference in days is returned.

Conversely, starting out from a given date, one can “jump” to a future or former date by addition or subtraction of an integer number, respectively. We use this possibility in the following problem definition: Statistically speaking, the 13th day of every month falls on a Friday more frequently than on any other day of the week. We want to check this statement with a program which iterates over all permitted dates of the class date, counting for every 13th day of a month the day of the week:

Filename: Friday13thCounter.C
```#include <LEDA/date.h>
#include <LEDA/array.h>
#include <iostream>

using leda::date;
using leda::array;

int main()
{
array<int> DOW_counter(1,7); // count Day Of Week
DOW_counter.init(0);

for (date D(1, date::Jan,1); ; D++)
{
if(D.get_day() == 13)
DOW_counter[D.get_day_of_week()]++;

// don't apply ++ to the last valid date:
if(D == date(31, date::Dec, 9999)) break;
}

for(int i = 1; i <= 7; i++)
std::cout << i << ": " << DOW_counter[i] << "\n";
}
```

The output of the program is

```1: 17123
2: 17124
3: 17173
4: 17097
5: 17199
6: 17099
7: 17173```

This shows that the 13th day of every month falls on the 5th day of the week, that is on Friday, more frequently than on any other day of the week. However, we should not infer from that that there are more unhappy days in life as happy ones, at least as long as we are young!

The operator ++ makes a date jump to the next day. The operator -- works correspondingly. The operators + and - allow to jump an arbitrary number of days from a given date into the future and back to the past, respectively.

The method

`D.get_day();`

returns the day in the month belonging to a date D, that is an integer number between 1 and 31.

The method

`D.get_day_of_week();`

returns the day in the week belonging to a date D, that is an integer number between 1 and 7.

#### Further information

The class date offers some further useful possibilities. For example, dates can be output or read with the help of a format string in almost every conceivable format. More information about this is found on the corresponding manual page.

#### Exercises

 Exercise 23. Which date was the day the author ran the above program to be informed of having already lived incredible 13252 days?