Dates and Times

1
Dates and Times

• How do we calculate dates and times?

2
Time

• Perl's time function returns the number of
  seconds that have passed since The Epoch (On Unix
  and many other systems, the Epoch was 0000 Jan
  1, 1970, GMT).

3
Time, in action

• the_time time
• print the_time, "\n"
• prints a huge number (number of seconds
• since The Epoch)
• years (((the_time/60)/60)/24)/365
• print years, "\n"
• prints number of years since The Epoch

4
More on time

• Epoch seconds represent intervals and points in
  the same units, so you can do basic arithmetic on
  them.
• However, people don't deal with epoch seconds we
  deal with weeks, months, etc.
• Because of the difficulty of performing
  calculations with a variety of formats, in Perl
  we might convert human-supplied dates
  (01/22/2001) to Epoch seconds, calculate, and
  then convert back to dates for output.

5
localtime (and gmtime)

• We usually want weeks, months, etc, not seconds.
  
• To convert epoch seconds into distinct values for
  days, months, years, hours, minutes, and seconds,
  use the localtime() function (use gmtime() if you
  want GMT).

6
localtime() and gmtime()

• NOTES
• Months are returned in the range 0-11
• Year is returned as number of years since 1900
• localtime() and gmtime() take an argument of
  seconds since the Epoch. The default is the
  current value of time

7
Quick Diversion on Years

• Perl does not return a 2-digit year value. It
  returns the year minus 1900.
• Y2K problem?? Only if you create one
• If you use a construct like
• full_year "19year"
• you'll find your scripts will refer to the year
  19101.
• Instead use
• year 1900

8
Y2038 problem

• However, there is (possibly) a Y2038 problem! If
  you have a 32-bit system, a 32-bit integer will
  hold your Epoch seconds. The range of this
  number is from December 1901 to January 2038.
  After that the Epoch seconds overflow 32 bits!

9
Localtime in action

• localtime() returns a bunch of info in
• list context
• my (seconds, minutes, hours, day, month,
  year, weekday, yearday, isdst) localtime
  (time)
• month why do we do this?
• year 1900 why do we do this?
• print "Date is month/day/year\n"

10
Cleaner Version of localtime

• just grab what you need using a slice
• (day, month, year) (localtime)3,4,5
• month
• year 1900
• printf comes in handy to make dates
• look clean
• printf "Date is 02d/02d/04d\n",
• month, day, year

11
Backwards

• Use TimeLocal to convert from day/month/year to
  an epoch time
• use TimeLocal
• time timelocal(0,0,0,day,
• mon - 1, year)
• Use localtime(number) to go from epoch to a
  day/month/year

12
Scalar form of localtime

• In scalar context, localtime() returns a
  human-readable string.
• This is often good enough if you don't need a
  particular date format (MM/DD/YYYY, DD-MM-YYYY,
  etc).
• date localtime
• print date
• prints something like
• Tues Apr 13 122931 1999

13
Modules for Dates

• There are several good modules on CPAN for
  dealing with dates and times. Here's two
• DateCalc
• DateManip

14
DateCalc

• Many functions in DateCalc. "perldoc
  DateCalc" for everything. Here's an example
• use strict
• use DateCalc 'Today_and_Now'
• my (year,month,day, hour,min,sec)
  Today_and_Now()
• printf "It is 02d02d02d on
• 02d/02d/04d\n",
• hour, min, sec, month, day, year

15
Another DateCalc example

• use strict
• use DateCalc 'Delta_Days'
• my _at_joel (1973, 3, 16) March 16, 1973
• my _at_sarah (1975, 9, 3) Sept 3, 1975
• my difference Delta_Days (_at_joel, _at_sarah)
• print "There were difference days between Joel
  and Sarah\n"

16
DateManip

• DateManip is the 800-pound gorilla of date
  manipulation modules biggest, most powerful, and
  slowest.
• Many functions are available. We will have a
  module presentation on this next quarter.
• It is designed to make any common date/time
  manipulation easy, not necessarily quickly.

17
Catching errors

• Many modern languages incorporate "exception
  handling"
• Java and C call them "throw" and "catch"
• Perls version is called die and eval

18
Catching errors

• Use eval to trap problems
• print "enter divisor"
• my divisor ltSTDINgt
• my result 100 / divisor
• catching the divide by zero error
• my result eval 100 / divisor

19
Catching errors

• You can look at the actual error caught in _at_
• sub getFile
• print "Enter file"
• my file ltSTDINgt
• open (FH, file) or die(!)
• more code
• eval getFile("filename.txt")
• if (_at_)
• print "There was an error opening file.\n"

20
Compiling code at run time

• Eval on a string (without ) is a completely
  different animal
• eval variable
• eval " code "
• This actually compiles code.
• Can be a security hole if hackers can run the
  program (like in a CGI).
• eval is safe.
• eval string can be dangerous.

21
Example

• print "Enter filename "
• my file ltSTDINgt
• eval
• open FH, file or die !
• versus
• eval "open FH, file or die !"

22
Process Management

• Usually theres a CPAN module that does what you
  need portably.
• But sometimes you want a quick-and-dirty script.

23
System

• system "dir"
• System starts a child process to run a shell
  command.
• Perl naps in the meantime
• STDOUT and STDIN are inherited

24
System (continued)

• Not portable
• Can be problems with user input.
• Whats wrong with this
• print "Enter filename"
• my file ltSTDINgt
• system("cat file")

25
Backticks

• Backticks run a system command and capture the
  output
• my help
• my _at_functions
• qw! int rand length not exit sqrt!
• foreach (_at_functions)
• help_ perldoc f _

26
Backticks (unix shell)

• the unix date command prints out the date
• in a variety of formats ("man date" for
• more info). Use backticks to run a shell
• command and capture the output, like so
• chomp(my date date 'm/d/Y')
• print date, "\n"
• prints MM/DD/YYYY