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Do you remember if this month ends on the 30th or the 31st?
Does September end on the 30th or the 31st?
The answer is the 30th.
Our life is becoming more convenient and it's easier to get information from devices like mobile phones and PCs. Although, what happens if your battery just died? Here is the Tip to find out what the last day of the month is. Use your knuckles! Count the numbers from right to left like shown in the image above. The high points of your knuckles are the months ending on the 31st and low points are the months ending on the 30th, with the exception of February. 
