you can use the expression:
sysdate - (columnA/(24*60))
If you are subtracting columnA from CurrentDateTime, it would mean that columnA is either a DateTime or an Offset from a DateTime. The subtraction would return the interval.
If columnA doesn't store an Offset from a particular DateTime, what would subtracting it from CurrentDateTime really mean ? What relation does it have to when it was inserted ?
Hemant K Chitale