Random Set of Dates for Laravel Project - php

Question

This is a Laravel project that I'm attempting to get a random collection of dates between the two dates below. The random dates chosen need to be 6 years apart and be on either a Monday, Thursday or Sunday. I have the dates function below that works for another date range I need however with this situation there's the additional factor of 6 years so I need the additional modification for it and not sure what I need to do to account for it on this situation.
$start = Carbon::parse('First Monday of January 2000');
$nextMonth = Carbon::now()->addMonth();
collect([
'monday' => false,
'thursday' => false,
'sunday' => true
])->flatMap(function ($bool, $day) use ($start, $nextMonth) {
return dates($start, $nextMonth, $day, $bool);
})->sort(function ($a, $b) {
return strtotime($a) - strtotime($b);
})->values()->map(function ($date, $key) {
return factory(Event::class)->create([
'name' => 'Event ' . ($key + 1),
'date' => $date
]);
})->filter(function ($event) {
return $event->date->lt(Carbon::today());
function dates(Carbon $from, Carbon $to, $day, $last = false)
{
$step = $from->copy()->startOfMonth();
$modification = sprintf($last ? 'last %s of next month' : 'next %s', $day);
$dates = [];
while ($step->modify($modification)->lte($to)) {
if ($step->lt($from)) {
continue;
}
$dates[$step->timestamp] = $step->copy();
}
return $dates;
}

Answer 1

If the second date is exactly six years from the first random date:
$second_date = $first_date->diffInYears($first_date->copy()->addYears(6))
From Carbon docs: http://carbon.nesbot.com/docs/#api-difference
Update:
Here is a way of creating an array of dates, based on a start date, where each date is at least six years apart and either a Monday, Thursday or Sunday.
I've left the loop at 20 iterations so you can see the dates generated are different days over the years.
$start = Carbon::parse('First Monday of January 2000');
$dates = array();
for ($i = 1; $i < 20; $i ++)
{
$interval = $i * 6;
if ($start->copy()->addYears($interval)->dayOfWeek === Carbon::MONDAY OR $start->copy()->addYears($interval)->dayOfWeek === Carbon::THURSDAY OR $start->copy()->addYears($interval)->dayOfWeek === Carbon::SUNDAY)
{
$dates[] = $start->copy()->addYears($interval);
} else
{
$dates[] = $start->copy()->addYears($interval)->modify('next monday');
}
}