How to sort numerically

In a report I have a column for "time until due". Overdue is shown with minus sign.

I want to sort this column, but the result is that it is sorted as text, e.g.

-73

-7

-5

-49

-47

-142

Can I, by using e.g. som conversion formula, make it sort numerically? I want the list above to be displayed as

-142

-73

-49

-47

-7

-5

I tried to use cdbl(sorting_field) but it didn't work.