With Date::Manip, if that is accepted, something like the following code might be considered simple and straightforward (read: most of the inherent underlying difficulties remain hidden inside Date::Manip)):

use warnings;
use strict;

use Date::Manip;

my $t = time();

my $date = ParseDateString( "epoch $t" );

my $day_start = Date_SetTime( $date, 0, 0, 0);
my $day_end   = Date_SetTime( $date, 23, 59, 59);

print "\nDay: ", start_end( $day_start, $day_end );

my $err;

my ($year, $month, $day, $day_of_week)
   = UnixDate( $date, '%Y','%m', '%d', '%w');

my $week_start = $day_of_week == 1 ? $date
               :                     DateCalc( $date, 'last Monday', \
+$err)
               ;
   $week_start = Date_SetTime( $week_start, 0, 0, 0);

my $week_end = $day_of_week == 7 ? $date
             :                     DateCalc( $date, 'next Sunday', \$e
+rr)
             ;
   $week_end   = Date_SetTime( $week_end, 23, 59, 59);

print "\nWeek: ", start_end( $week_start, $week_end );

my $month_start = Date_SetDateField( $date, 'D', 1 );
   $month_start = Date_SetTime( $month_start, 0, 0, 0);
                                   
my $next_month      = DateCalc( $date, 'next month' );
my $month_end_epoch = UnixDate( $next_month, '%s' ) - 1;

my $month_end = ParseDateString( "epoch $month_end_epoch" );
   $month_end = Date_SetTime( $month_end, 23, 59, 59);

print "\nMonth: ", start_end( $month_start, $month_end );

my $year_start = Date_SetDateField( $date, 'M', 1);        # January
   $year_start = Date_SetDateField( $year_start, 'D', 1 ); # 1
   $year_start = Date_SetTime( $year_start, 0, 0, 0);

my $year_end = Date_SetDateField( $date, 'M', 12);      # December
   $year_end = Date_SetDateField( $year_end, 'D', 31 ); # 31
   $year_end = Date_SetTime(      $year_end, 23, 59, 59 );

print "\nYear: ", start_end( $year_start, $year_end );

exit;

sub start_end {
    my ($date_start, $date_end) = @_;

    return join ( '', 'starts at ',
                      UnixDate( $date_start, '%O'),
                      ', ends at ',
                      UnixDate( $date_end, '%O'),
                      "\n\t(",
                      UnixDate( $date_start, '%s'),
                      ' - ',
                      UnixDate( $date_end, '%s'),
                      ")\n",
                );
}
[download]

which produces output like

Day: starts at 2007-11-04T00:00:00, ends at 2007-11-04T23:59:59
        (1194127200 - 1194213599)

Week: starts at 2007-10-29T00:00:00, ends at 2007-11-04T23:59:59
        (1193608800 - 1194213599)

Month: starts at 2007-11-01T00:00:00, ends at 2007-11-30T23:59:59
        (1193868000 - 1193954399)

Year: starts at 2007-01-01T00:00:00, ends at 2007-12-31T23:59:59
        (1167602400 - 1199138399)
[download]

A slight simplification is done anyway: timezone of the starting epoch is considered to be identical to the one in which the results are given.

Hope that helps.

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