Leave cell blank if referenced cell and/or formula is blank or incomplete

The Dates in Process column is populated by subtracting the Date created from the completed date in order to get the total number of days. However when an item is not completed and there is no completed date, the Dates in Process is populated with a -4thousand number . I would want for the cell to be blank if there is no completed date entered in that row.
That goes the same for the lead time which is computed by subtracting the Date created from the InWork Date (gets crazy when there is no Inwork Date.

Also, for future reference I created a column that extracts the month in which an item was completed (from Completed Date Column) using =TEXT([@[Completed Date]],"mmmm")


However, when there is no Completed Date the Month displayed is January , which should be blank. How can I fix these things?