The FIND function will determine the character where the 3 letters representing the month can be found, so it will return 1 for Jan, 4 for Feb, 7 for Mar, and so on. 1 is subtracted from this number (giving 0, 3, 6 etc. for those months), and then this is divided by 3 to return 0, 1, 2 through the INT function, and then 1 is added back in, so those months will return 1, 2, 3 and so on. That part of the formula thus returns the month number represented by the 3-letter month.

Hope this helps.

Pete