Added function to get the date of a sub-booking.

app/SubBooking.php

@@ -6,5 +6,13 @@ use Illuminate\Database\Eloquent\Model;
 
 class SubBooking extends Model
 {
-    //
+     /*
+      * Get the booking date for a sub-booking.
+      *
+      * @return string
+      */
+     public function date()
+     {
+         return $this->belongsTo('\App\Booking', 'booking_id')->first()->date;
+     }
 }