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Your CS order should rule on this. For instance, if it says $X every month,payable on the 1st of the month, until she turns 18, then you would arguably pay the full $X since she was not yet 18 on the 1st. If is states a weekly amount, go strictly with that; if it says biweekly, I would pay the biweekly amount due on or before her birthday,but likely not the bi-weekly date after that.
If it fails to state, you may have an argument for pro-ration - I can hardly imagine that the ex will take you to court over such a small amount of money, particularly if she is not guaranteed a win. However, if you are paying through CS agency (which it doesn't appear), some agencies interpret this as meaning through the whole month, regardless, and don't prorate. However, I would be careful not to stretch your interpretation of your order if it is not readily and reasonable to read it that way, so try to read it unemotionally and objectively.
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