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there are several ways to construct calculation you require, here is one which doesn't require advanced options:
CurrentDate() is available as a replacement for get_date() both as Pre-Defined Formula Type and as Freehand SQL Formula Type (the latter may be disabled in your Smart Reporting installation), so you can use that to define first calculated field.
After this, define two more calculated fields:
one (Until NOW) to calculate Current_Date - Reported Date, another (Until Closing) to calculate Closed Date - Reported Date
and use them both in final calculated field as CASE WHEN Closed Date IS NULL THEN Until NOW ELSE Until Closing END.