Notes: Perl Lab 11

Date and Time Functions

  1. time
  2. localtime
  3. modules
  4. timelocal

  1. time

     time simply returns the current system time as number of seconds that have elapsed since the time that the system regards as the "epoch".For most Unix based systems this is 12:00 AM Jan 1, 1970 UTC.
    $seconds = time;

  2. localtime

     By default, the localtime function returns the current time. In scalar context this will be the time formatted as dictated by the system's locale settings. This syntax generally will take on the form:
    $now = localtime();
    Which is the same as the following:
    $now = localtime(time);
    On our systems $now will have a value that looks something like this:
    Sat Apr 24 13:01:53 2004
    Example lab11_0.pl
      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab11 Example 00
# Description: An example of the time and localtime functions
#==========================================================================

# show that the time function returns an integer
$curr_time = time;
print "Current time in seconds is $curr_time\n";


# print the output  in scalar context.  Notice that the contantenation within 
# the print function tells the perl interpreter that localtime output is in
# a scalar context.
print 'One: '. localtime(time) . "\n";
print 'Two: '. localtime() . "\n";


# another example of localtime in scalar context
$time_string = localtime();
print "Current time is: $time_string\n";


# we can get the time information from another date by manipulating the number
# of seconds as the argument to localtime.  Here we subtract one day (in 
# seconds) from the current time and use that value with the localtime func.
$yesterday = $curr_time - 86400;
print "Yesterday was " . localtime($yesterday) ."\n";
exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@platypus PERL] ./lab11_0.pl  
Current time in seconds is 1083001729
One: Mon Apr 26 17:48:49 2004
Two: Mon Apr 26 17:48:49 2004
Current time is: Mon Apr 26 17:48:49 2004
Yesterday was Sun Apr 25 17:48:49 2004
[mark@platypus PERL]

    localtime may be more commonly used in list context. In this case, localtime returns an array containing various values of the time. Below is a table of the returned values in the order that the function will return them and what their meaning:
    Index Units Description
    0secondsValues range 0-59
    1minutesValues range 0-59
    2hoursValues range 0-23
    3day of monthValues range 1-31 depending on the month
    4monthValues range 0-11 where 0 is January and 11 is December.
    5yearReturnrs the number of years since 1900. For example, 2004 would have a value of 104.
    6weekdayReturns the number of the day of the week 0-6 for Sunday-Saturday
    7julian dayReturns 0-364 for non-leap years and 0-365 for leap years
    8daylight savings timeReturns true or "1" if the time occurs during daylight savings time. Otherwise it returns 0 or false.
    Example lab11_1.pl
      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab11 Example 01
# Description: An example of the localtime function in list context
#==========================================================================

# Get the current time in seconds and call the Display_time function to
# display the values.
$now = time;
print "Displaying raw current time\n";
&Display_time($now);


# subtract 120 days from the current time and get the localtime values and 
# then call the Display_time function to display the values.
$new_time = time - (60*60*24*120);
print "\nDisplaying raw time 120 days ago\n";
&Display_time($new_time);

exit;
# DONE


sub Display_time {
    # rename the function's arguments
    my $time_arg = $_[0];

    # Get the current time
    ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = 
        localtime($time_arg);

    # print out the raw values:
    print "Seconds are: $sec\n".
        "Minutes are: $min\n".
        "Hours are: $hour\n".
        "Day of month: $mday\n".
        "Month is: $mon\n".
        "Year is: $year\n".
        "Week day is: $wday\n".
        "Julian day is: $yday\n".
        "EST/EDT is: $isdst\n";

    if($isdst) {
        print "It is Daylight Savings Time\n";
    }else{
        print "It is not Daylight Savings Time\n";
    }
}

    When executed, the script above produces the following output:
      
    [mark@platypus PERL] ./lab11_1.pl  
Displaying raw current time
Seconds are: 48
Minutes are: 0
Hours are: 18
Day of month: 26
Month is: 3
Year is: 104
Week day is: 1
Julian day is: 116
EST/EDT is: 0
It is not Daylight Savings Time

Displaying raw time 180 days ago
Seconds are: 48
Minutes are: 0
Hours are: 18
Day of month: 28
Month is: 11
Year is: 103
Week day is: 0
Julian day is: 361
EST/EDT is: 0
It is not Daylight Savings Time
[mark@platypus PERL]

    Example lab11_2.pl
      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab11 Example 02
# Description: An example of correcting the return values from the
#              localtime function
#==========================================================================

# get the time values
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time);

# correct the date values to match how humans generally count things.
$mon++;
$year += 1900;
$wday++;
$yday++;

# Show the new values 
print "Seconds are: $sec\n".
    "Minutes are: $min\n".
    "Hours are: $hour\n".
    "Day of month: $mday\n".
    "Month is: $mon\n".
    "Year is: $year\n".
    "Week day is: $wday\n".
    "Julian day is: $yday\n".
    "EST/EDT is: $isdst\n";

# Is it daylight savings time?
if($isdst) {
    print "It is Daylight Savings Time\n";
}else{
    print "It is not Daylight Savings Time\n";
}

    When executed, the script above produces the following output:
      
    [mark@platypus PERL] ./lab11_2.pl  
Seconds are: 42
Minutes are: 5
Hours are: 18
Day of month: 26
Month is: 4
Year is: 2004
Week day is: 2
Julian day is: 117
EST/EDT is: 0
It is not Daylight Savings Time
[mark@platypus PERL]

    You may not want to alter all the values but rather, use them as indexes for other arrays.

    Example lab11_3.pl
      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab11 Example 03
# Description: An example of using localtime values
#==========================================================================

# setup some arrays of text values
@months = qw(Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov);
@weekday = qw(Sunday Monday Tuesday Wednesday Thursday Friday Saturday);

# get the current time values
@now = localtime(time);

# set the string values based on the array values
$current_month = $months[$now[4]];
$current_weekday = $weekday[$now[6]];

# print the values to the screen
print "The month is $current_month\n";
print "The day of the week is $current_weekday\n";

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@platypus PERL] ./lab11_3.pl
The month is Apr
The day of the week is Monday
[mark@platypus PERL]

  3. modules

     Before moving on to the functions availble in the Time::Local module, the inclusion of modules should be discussed. The function use is used to make the specified Perl module available to the program. Generally, this will make new functions available to the program.
    use Time;
    This will make functions within the Time module available for use in the current script. Specific versions or routes of the module may be specified by appending the appropriate name to the module name separated by a double colon "::"
    use Time::Local;

  4. timelocal

     The Time::Local module makes some additional functions available for manipulating time values. In particular, the timelocal function will be examined.

    timelocal, like its name, is the inverse of the localtime function. timelocal expects a list of date values provided to it as arguments and returns an integer time value for the number of seconds since the epoch.

    $seconds = timelocal($sec,$min,$hour,$mday,$mon,$year);
    This is particularly useful when there is the need to manipulate date values other than the current date and time. Those values can be converted to the number of seconds since the epoch through the use of timelocal. Once the values are in seconds units the date data can then be manipulated mathematically much more easily.

    Example lab11_4.pl
      
    #!/usr/bin/perl -w

# Name: Mark Tucker
# Assignment: Lab11 Example 04
# Description: An example of the timelocal.  Given the date from user input
#              calculate the date three days prior and display on the screen.
#==========================================================================

# include the Time::Local module
use Time::Local;

# get the date information from the user
print "Enter a year value: ";
chomp($user_year = <STDIN>);

print "Enter a month value (0-11): ";
chomp($user_month = <STDIN>);

print "Enter a day value (0-31): ";
chomp($user_day = <STDIN>);


# convert the current date to seconds and print to the screen
$user_date = timelocal(0,0,0,$user_day,$user_month,$user_year);
$user_formatted = localtime($user_date);
print "The input date is $user_formatted\n";

# subtract 3 days from the input date and print to the screen
$past_seconds = $user_date - (60*60*24*3);
$past_formatted = localtime($past_seconds);
print "The date 3 days prior to the input date is $past_formatted\n";

exit;
# DONE

    When executed, the script above produces the following output:
      
    [mark@platypus PERL] ./lab11_4.pl
Enter a year value: 2004
Enter a month value (0-11): 2
Enter a day value (0-31): 15
The input date is Mon Mar 15 00:00:00 2004
The date 3 days prior to the input date is Fri Mar 12 00:00:00 2004
[mark@platypus PERL]

last updated: 18 Mar 2012 12:58