Change Your calendar starting date based on your availability rule.

function.php

/**
 * Change Your calendar starting date based on your availability rule.
 * Created at : 27th June 2019
 */  
add_filter( 'ph_booking_calendar_start_date', 'ph_booking_calendar_start_date',10, 2 );
function ph_booking_calendar_start_date($start_date,$product_id)
{
	// return $message;
		$fixed_availability_from 	= get_post_meta( $product_id, "_phive_fixed_availability_from", 1 );
		$fixed_availability_to 	= get_post_meta( $product_id, "_phive_fixed_availability_to", 1 );
		$availabiliy_rules				= get_post_meta( $product_id, '_phive_booking_availability_rules', 1 );
		$un_availabiliy				= get_post_meta( $product_id, '_phive_un_availability', 1 );
		$availabiliy_rules = !empty($availabiliy_rules) ? $availabiliy_rules : array();
		$availability_dates=array();
		$calendar_for='date-picker';
		$min_avail_date=9999999999;
		foreach ($availabiliy_rules as $key => $rule) {
			if( $rule['availability_type']=='custom' ){
				if($calendar_for == 'time-picker' || $calendar_for == 'date-picker'){
					$date_from = explode(" ",$rule['from_date']);
					$date_from = $date_from[0];
					$date_to = explode(" ",$rule['to_date']);
					$date_to = $date_to[0];
				}
				else{
					$date_from  = $rule['from_date'];
					$date_to  = $rule['to_date'];
				}
				$date_from_str=strtotime( $date_from );
                $availability_dates[$date_from] = isset($availability_dates[$date_from])?$availability_dates[$date_from]:$rule['is_bokable'];
               	$min_avail_date=($min_avail_date>$date_from_str && $rule['is_bokable']=='yes')?$date_from_str:$min_avail_date;
                while( strtotime( $date_from ) <= strtotime( $date_to ) )
                {
                    $date_from = date( 'Y-m-d',strtotime( "+1 day", strtotime($date_from) ) );
                    $date_from_str=strtotime($date_from);
                    $min_avail_date=($min_avail_date>$date_from_str && $rule['is_bokable']=='yes')?$date_from_str:$min_avail_date;
                    $availability_dates[$date_from] = isset($availability_dates[$date_from])?$availability_dates[$date_from]:$rule['is_bokable'];
                }

			}
		}
		ksort($availability_dates);
		$timezone = get_option('timezone_string');
		if( empty($timezone) ){
			$time_offset = get_option('gmt_offset');
			$timezone= timezone_name_from_abbr( "", $time_offset*60*60, 0 );
		}
		date_default_timezone_set($timezone);
		$current_date=strtotime(date('Y-m-1'));
		// Start date
		if(  !empty($fixed_availability_from) &&  strtotime($fixed_availability_from) >= $current_date ){
			$start_date = date( 'Y', strtotime($fixed_availability_from) ).'-'.date( 'm', strtotime($fixed_availability_from) ).'-01';
		}else{
			if($un_availabiliy=='no')
			{
				$today=date('Y-m-d');
				if(isset($availability_dates[$today]) && $availability_dates[$today]!='yes')
				{
					foreach ($availability_dates as $date => $status) {
						if(strtotime($today) >= strtotime($date) )
						{
							continue;
						}
						else
						{
							$next_day=date('Y-m-d', strtotime('+1 day', strtotime($today)));
							if(strtotime($next_day)!=strtotime($date))
							{
								$start_date=date('Y-m-d',strtotime($next_day));
								$start_date = date( 'Y', strtotime($start_date) ).'-'.date( 'm', strtotime($start_date) ).'-01';
							}
							else
							{
								$today=$next_day;
							}
						}
					}
				}
				else
				{
					$start_date =date('Y').'-'.date('m').'-01';
				}
			}
			else if($min_avail_date!=9999999999)
			{
				$start_date=date('Y-m-d',$min_avail_date);
				$start_date = date( 'Y', strtotime($start_date) ).'-'.date( 'm', strtotime($start_date) ).'-01';
				// $start_date = date('Y').'-'.date('m').'-01';
			}
			else
			{
				$start_date = date('Y').'-'.date('m').'-01';	
			}
			
		}
	return $start_date;
}