Contract.php

<?php

namespace App;

use Illuminate\Database\Eloquent\Model;

class Contract extends Model
{
    public $timestamps = false;

    protected $casts = [
        'valid_range' => \App\Casts\DateRangeCast::class,
    ];

    public function newEloquentBuilder($query): ExtendedQueryBuilder
    {
        return new ExtendedQueryBuilder($query);
    }
}

DateRange.php

<?php

namespace App;

use Carbon\CarbonImmutable;
use Carbon\CarbonInterface;

class DateRange
{
    private ?CarbonInterface $from;

    private ?CarbonInterface $to;

    public function __construct($from = null, $to = null, $fromBound = "[", $toBound = "]")
    {
        $this->from = is_string($from) ? $this->parseFrom($from) : $from;
        $this->to = is_string($to) ? $this->parseTo($to) : $to;

        // when exclusive bound is set,
        // let's canonicalize it to inclusive bounds
        if ($fromBound === '(') {
            $this->from = $this->from->addDay();
        }

        if ($toBound === ')') {
            $this->to = $this->to->subDay();
        }
    }

    private function parseFrom(string $from) : CarbonInterface
    {
        return CarbonImmutable::parse($from);
    }

    private function parseTo(string $to) : CarbonInterface
    {
        return CarbonImmutable::parse($to);
    }

    public function from(): ?CarbonInterface
    {
        return $this->from;
    }

    public function to(): ?CarbonInterface
    {
        return $this->to;
    }
}

DateRangeCast.php

<?php

namespace App\Casts;

use Illuminate\Contracts\Database\Eloquent\CastsAttributes;

class DateRangeCast implements CastsAttributes
{
    public function get($model, $key, $value, $attributes)
    {
        preg_match('/([\[\(]{1})(.*)\,(.*)([\]\)]{1})/', $attributes['valid_range'], $matches);

        return new \App\DateRange($matches[2], $matches[3], $matches[1], $matches[4]);
    }

    public function set($model, $key, $value, $attributes)
    {
        return [
            'valid_range' => $this->serializeRange($value)
        ];
    }

    private function serializeRange($range)
    {
        return "[" .
            optional(optional($range)->from())->toDateString() .
            "," .
            optional(optional($range)->to())->toDateString() .
            "]";
    }
}