The day function is used to find day from a date,for example, the Day Function return 20 from 20/Jan/2020
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You can get total number of days of each month by using DAY function and EOMONTH Function
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If you find a data with date column in which date has been stored as shown below
05.06.2010 it is read as June 05, 2010, but it can also be read as May 06, 2010. The correct one is June 05, 2010 according to my regional settings.
If you replace the full stop sign with slash sign,it is possible that the date is converted to May 06,2010 which is incorrect answer.
In this scenario you can use DATE Function along with LEFT, RIGHT and MID Function to solve the problem.
Read more DATE Function