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NOT
Returns FALSE for TRUE and TRUE for FALSE.
Use
NOT(logical) and replace
logical with the expression that you want evaluated.
IF(NOT(ISPICKVAL(Status, "Closed")), ROUND(NOW()-CreatedDate, 0),
null checks to see if a variable is open and if so, calculates the number of days it
has been open by subtracting the date and time created from the current date and time. The
result is the number of days open rounded to zero decimal places. If the variable is not open,
this field is blank.

