Programming in C

192 Chapter 9 Working with Structures

days between July 2, 2005, and July 16, 2005, is obviously 14. But how many days

are there between August 8, 2004, and February 22, 2005? This calculation requires

a bit more thought.

Luckily, a formula can be used to calculate the number of days between two dates.

This is affected by computing the value of Nfor each of the two dates and then

taking the difference, where Nis calculated as follows:

N = 1461 x f(year, month) / 4 + 153 x g(month) / 5 + day

where

f(year, month) = year - 1 if month <= 2

year otherwise

g(month) = month + 13 if month <= 2

month + 1 otherwise

As an example of applying the formula, to calculate the number of days between

August 8, 2004, and February 22, 2005, you can calculate the values of N 1 and N 2

by substituting the appropriate values into the preceding formula as shown:

N 1 = 1461 x f(2004, 8) / 4 + 153 x g(8) / 5 + 3

= (1461 x 2004) / 4 + (153 x 9) / 5 + 3

= 2,927,844 / 4 + 1,377 / 5 + 3

= 731,961 + 275 + 3

= 732,239

N 2 = 1461 x f(2005, 2) / 4 + 153 x g(2) / 5 + 21

= (1461 x 2004) / 4 + (153 x 15) / 5 + 21

= 2,927,844 / 4 + 2295 / 5 + 21

= 731,961 + 459 + 21

= 732,441

Number of elapsed days = N 2 - N 1

= 732,441 – 732,239

= 202

So the number of days between the two dates is shown to be 202.The preceding

formula is applicable for any dates after March 1, 1900 (1 must be added to Nfor

dates from March 1, 1800, to February 28, 1900, and 2 must be added for dates

between March 1, 1700, and February 28, 1800).

Write a program that permits the user to type in two dates and then calculates the

number of elapsed days between the two dates.Try to structure the program logi-

cally into separate functions. For example, you should have a function that accepts

as an argument a datestructure and returns the value of Ncomputed as shown