Calculating cron next-run-time in PHP

Question

I'm designing a task scheduler in my own personal framework and am trying to avoid the not-as-flexible "run every n minutes/hours/days" method that'd be easier to implement. What I'd like to do is mimic cron scheduling. I've got functions in place to split patterns and calculate the next value for the next date (day of month) currently, but don't want to keep pushing forward if there's something easier than what I'm doing, or possibly a better way to do what I'm trying to do.

/**
  * Takes pattern(s) for various time attributes and calculates the next time the task should run
  *
  * @param mixed $minute Pattern or value of minute: 0-59 or 15,45 or * or * /5 (every 5 minutes)
  * @param mixed $hour Pattern or value of hour: 0-23 or 0,6,12 or * or * /2 (every 2 hours)
  * @param mixed $date Pattern or value of date: 1-31 or 1,15 or * or * /5 (every 5 days)
  * @param mixed $day Pattern or value of weekday: 0-7 or 0,1,7 or * or * /7 (every 7 days) - takes precedence over date
  * @return string Timestamp of next run time
  */
 public static function calcNextRun( $minute, $hour, $date, $day )
 {
  # Simplest first, if all * then we run every minute. Return timestamp for next whole minute
  if ( $minute == '*' && $hour == '*' && $date == '*' && $day == '*' )
   return mktime( date( "H" ), date( "i" ), 0 ) + 60; # Prettier than time() + 60, isn't that reason enough?

  # Default to current values
  $nextDate = date( "d" );
  $nextMonth = date( "m" );
  $nextYear = date( "Y" );
  $nextDay = date( "N" );
  $nextHour = date( "H" );
  $nextMinute = date( "i" );

  # Calculate month date to run on, using multiple dates in the presence of , or -
  if( strstr( $date, ',' ) || strstr( $date, '-' ) )
  {
   # Variable to determine whether the date has been set or not
   $dateSet = false;

   # Determine if there's a range in thurr
   $rangeExists = ( strstr( $date, '-' ) ) ? true : false ;

   # Set up the $dates array, exploding if multiple values is present
   $dates = array();
   if ( strstr( $date, ',' ) )
    $dates = explode( ',', $date );
   else
    $dates[] = $date;

   # If we have a range(s) present then we expand them into full stuffs
   foreach ( $dates as $key => $val )
    if ( strstr( $val, '-' ) )
     $dates = array_merge( $dates, self::expandRange( $val ) ); # Merge the expanded range into the $dates array

   # Loop through the $dates array and remove any lingering ranges
   foreach ( $dates as $key => $val )
    if ( strstr( $val, '-' ) )
     unset( $dates[ $key ] );

   # Sort the array
   sort( $dates );

   # Determine the next lowest value
   foreach( $dates as $val )
   {
    # If the value is higher than the maximum number of dates this month, lower it to that
    if ( $val > date( "t" ) )
     $val = date( "t" );

    # If $val is higher than today's date, we use that
    if ( $val > date( "d" ) )
    {
     $nextDate = $val;
     $dateSet = true;
     break; # We're done, we have our value
    }
   }

   # If the date has not been set, add one to the month and use the lowest value in the array
   if ( !$dateSet )
   {
    # Increment the month. Maybe the year. Hurr hurr
    if ( $nextMonth == 12 )
    {
     $nextMonth = 1;
     $nextYear++;
    }
    else
     $nextMonth++;

    # Set the next day to the lowest value in the array
    $nextDate = $dates[0];
   }
  }
  elseif ( strstr( $date, '/' ) ) # Every n days
  {
   $parts = explode( '/', $date );
   $numDays = array_pop( $parts );

   # Calculate the timestamp of n days from now
   $nDayTime = time() + ( $numDays * 86400 ); # 86400 seconds in a day

   # Update values of $nextVars
   $nextDate = date( "d", $nDayTime );
   $nextMonth = date( "m", $nDayTime );
   $nextYear = date( "Y", $nDayTime );
   $nextDay = date( "N", $nDayTime );
  }
  elseif ( $date == (int)$date )
  {
   if ( $date < date( "j" ) )
   {
    # Determine if the month pushes into the next year
    if ( $nextMonth == 12 )
    {
     $nextMonth = 1;
     $nextYear++;
    }
    else
     $nextMonth++;
   }

   $nextDate = $date;
  }

  # Return the new timestamp!
  return mktime( $nextHour, $nextMinute, 0, $nextMonth, $nextDate, $nextYear );
 }

 /**
  * Takes a range and returns an array with all values belonging to that range
  *
  * @param string $range Two values split by a hyphen, ie: 1-5, 0-9, etc.
  * @return array Array of values between the two parts of the range
  */
 private static function expandRange( $range )
 {
  # Get the parts of the range
  $range = explode( '-', $range );

  # Sort just in case the range is handed to us backwards. <_<
  sort( $range );

  # Set up our return array
  $returnArray = array();

  # Populate the return array with all values between min and max
  for($i=$range[0];$i<=$range[1];$i++)
   $returnArray[] = $i;

  return $returnArray;
 }

I don't mind using all five of the parameters that cron uses, but either way I want to be able to easily calculate the next timestamp that matches the pattern provided.

Does anyone have any recommendations for accomplishing this? I'm thinking of creating a function that will take a pattern (1-7,10,15 or */5 or * or whatever) and a current val (the current minute, day of month, whatever) and return the next value from that pattern that either matches or is higher than the current value.