I set a column to be date format YYYY-MM-DD but if I put in July 14, 2015 it doesn’t convert it. What am I doing wrong?
You need to enter the date in a format that Calc will recognise as a date - starting with July causes Calc to interpret the input as text.
See sections 2 and 3 of this article for more information.
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“starting with July causes Calc to interpret the input as text” actually is not true, but it depends on the locale what strings are recognized as dates. In an English-US locale July 14, 2017 is perfectly recognized as date, but in a non-English locale of course it is not.