Query Date Math
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Many Query users want to do date math, where we find out how many days ago some date was, or we select data based on including stuff for the last 7 days, and we not want to force the user to always have to key in whatever cut-off date to use. We would rather key in how many days to include, and let Query do the date math.
There are certain kinds of Query operations that can only be done on certain Query Field Types so it is often neccessary to convert data between field types to get the desired actions.