Question

I have a tab delimited file where each record has a timestamp field in 12-hour format:

mm/dd/yyyy hh:mm:ss [AM|PM].

I need to quickly convert these fields to 24-hour time:

mm/dd/yyyy HH:mm:ss.

What would be the best way to do this? I’m running on a Windows platform, but I have access to sed, awk, perl, python, and tcl in addition to the usual Windows tools.

Answer 1

Using Perl and hand-crafted regexes instead of facilities like strptime:

#!/bin/perl -w while (<>) {     # for date times that don't use leading zeroes, use this regex instead:     # (?:\d{1,2}/\d{1,2}/\d{4} )(\d{1,2})(?::\d\d:\d\d) (AM|PM)     while (m%(?:\d\d/\d\d/\d{4} )(\d\d)(?::\d\d:\d\d) (AM|PM)%)     {         my $hh = $1;         $hh -= 12 if ($2 eq 'AM' && $hh == 12);         $hh += 12 if ($2 eq 'PM' && $hh != 12);         $hh = sprintf '%02d', $hh;         # for date times that don't use leading zeroes, use this regex instead:         # (\d{1,2}/\d{1,2}/\d{4} )(\d{1,2})(:\d\d:\d\d) (?:AM|PM)         s%(\d\d/\d\d/\d{4} )(\d\d)(:\d\d:\d\d) (?:AM|PM)%$1$hh$3%;     }     print; } 

That’s very fussy – but also converts possibly multiple timestamps per line.

Note that the transformation for AM/PM to 24-hour is not trivial.

• 12:01 AM –> 00:01
• 12:01 PM –> 12:01
• 01:30 AM –> 01:30
• 01:30 PM –> 13:30

Now tested:

perl ampm-24hr.pl <<! 12/24/2005 12:01:00 AM 09/22/1999 12:00:00 PM 12/12/2005 01:15:00 PM 01/01/2009 01:56:45 AM 12/30/2009 10:00:00 PM 12/30/2009 10:00:00 AM !  12/24/2005 00:01:00 09/22/1999 12:00:00 12/12/2005 13:15:00 01/01/2009 01:56:45 12/30/2009 22:00:00 12/30/2009 10:00:00 

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Added:

In What is a Simple Way to Convert Between an AM/PM Time and 24 hour Time in JavaScript, an alternative algorithm is provided for the conversion:

$hh = ($1 % 12) + (($2 eq 'AM') ? 0 : 12); 

Just one test…probably neater.