Smalltalk and Object Orientation: an Introduction - Free

14.3 The class Date
Class Date, a subclass of Magnitude, represents a particular day, in a particular month of a
particular year. A date object can be created in a number of ways . For example, it is possible to request
the date today and thus create a date object:
todaysDate := Date today.
It is also possible to create a date object for an explicit date, using the
For example:
newDay:month:year:.
birthDate := Date newDay: 23 month: #Sep year: 1964.
Note that only the first three letters of the month name are significant and that if the century is not
provided in the year parameter (e.g. if 64 had been passed in rather than 1964), then the current century
is used as a default.
The Date class also allows date objects to be compared, added, subtracted etc. by virtue of its
inherited abilities from Magnitude. For example:
(todaysDate < birthDate) ifTrue: [Transcript show: 'Weird!'].
Transcript show: (todaysDate subtractDate: birthDate) printString.
This will calculate the difference between today’s date and the 23rd of September 1964. The result
will be printed in terms of the difference in days. To convert this into (an approximate) number of years,
you can divide by 365. This ignores leap years. For example:
Transcript show: ((todaysDate subtractDate: birthDate) // 365).
Question: Why have I used // rather than / ? Answer, because / will return a fraction if the number of
days cannot be divided exactly by 365, where as // will ignore any remainder.
There are also a range of other messages which are supported by the class Date. These messages
allow you to determine if the date object is in a leap year, which day of the year it is, which month the
date object is in, which year it is part of etc. Some examples using todaysDate are presented below:
todaysDate day.
todaysDate leap.
todaysDate monthName.
todaysDate weekday.
todaysDate year.
Class Date also supports a range of messages itself. These messages provide additional behavio r
related to dates. For example, the Date class will respond to messages requesting the number of days
in a particular month for a particular year:
Date daysInMonth: #Feb forYear: 1997.
Another class method can be used to determine if a given year is a l
using the leapYear: message to the Date class:
eap year or not. This is done
Date leapYear: 1999.
14.4 The class Time
The class Time, provides a similar set of features for objects which represent a specific instant of time
(to a particular second) as the class Date does for a specific calendar date. A new instance of the class
Time can be created using the instance creation message now or by reading from a string. The string
can contain hours, minutes and seconds separated by colons (‘11:59:20’). It can also contain an AM or
PM designation. For example:
currentTime := Time now.
previousTime := Time readFromString: '1:10:20 am'.
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