#!/usr/bin/perl

use POSIX;

my $cmdstr="%Y-%m-%d %H:%M:%S";
my ($input, $arg, $sign)=(undef, undef, 0);
my ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)=localtime();

# Commandline parsing stuff
while(defined($arg=shift)) {
        if($arg =~ /^--date=/)  {       $input=substr($arg, 7);         }
        elsif($arg =~ /^-d$/)   {       $input=shift;                   }
        elsif($arg =~/^\+/)     {       $cmdstr=substr($arg,1);         }
        else                    {       print STDERR "unknown $arg\n";  }
}

# Put the date string back into argv, split on spaces
unshift(@ARGV, split(/[ \t]+/, $input));

while(defined($arg=shift))
{
        # Need to split +1 into +1
        if($arg =~ /^[+]/) {    $sign=1;        $arg=substr($arg,1);    }
        elsif($arg =~ /^-/) {   $sign=-1;       $arg=substr($arg,1);    }

        ################## DATE FORMAT DETECTION ########################

        # @1234 means seconds in epoch time
        if($arg =~ /^@[0-9]+$/)
        {
                ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)=localtime($1+0);
                next;
        }

        # Checks for YYYYMMDD or YYYY/MM/DD time
        # TODO:  Check for YYMMDD dates
        # TODO:  Check for YYYYDDMM dates (ugh)
        if($arg =~ /^([0-9]{4})(\/?)([0-9]{2})\2([0-9]{2})/)
        {
                ($year,$mon,$mday)=($1-1900,$3-1,$4+0);
                ($sec,$min,$hour)=(0,0,0);
                next;
        }

        # HH:MM:SS times
        if($arg =~ /([0-2][0-9]):([0-5][0-9]):([0-5][0-9])(.[0-9]+)?$/)
        {
                ($sec,$min,$hour)=($3+0, $2+0, $1+0);
                next;
        }

        # As last resort, assume its a pure number.
        if($arg =~ /^([0-9]+)$/) {
                if($sign == 0)
                {
                        print STDERR "offset without unit";
                        exit(1);
                }

                $offset=$1+0;
                next;
        }

        if($arg =~ /^seconds?$/)        {       }
        elsif ($arg =~ /^years?$/) {
                                        $year += ($offset*$sign);
                                        $sign=0;
                                        $offset=0;
                                }
        elsif($arg =~ /^minutes?$/)     {       $offset *= 60;          }
        elsif($arg =~ /^hours?/)        {       $offset *= 60*60;       }
        elsif($arg =~ /^days?/)         {       $offset *= 60*60*24;    }
        elsif($arg =~ /^weeks?/)        {       $offset *= 60*60*24*7;  }
        elsif($arg =~ /^months?$/)      {

                                        $mon += ($offset*$sign);

                                        while($mon > 12)
                                        {
                                                $mon-=12;
                                                $year++;
                                        }

                                        while($mon < 0)
                                        {
                                                $mon+=12;
                                                $year--;
                                        }

                                        $sign=0;
                                        $offset=0;
                                }
        elsif($arg =~ /^now$/)  {
                ($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)=localtime();
        }
        elsif(length($arg) == 0){       } # Empty string?  Ignore
        else                    {
                                        print STDERR "Unknown argument $arg\n";
                                        exit(1);
        }
}

# Convert the altered year, month, etc back into epoch time.
my $ref=mktime($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst);
# Add the seconds offset calculated above.
$ref += ($sign * $offset);
# Convert back into list.
($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst)=localtime($ref);

print strftime($cmdstr."\n",
        $sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst);