Excel formula to stop auto date calculation when the required date has been reached

Hi,
I am composing a spread sheet, in column "A" i manually put in start date, Column "B" has the Now() formula for auto date update, column "c" has off hire date put in manually. In Column "d" if put in Datedif(start_date,End_date,"d") which tells me the number of days the item has been on hire. What i want is for column "B" now() to stop autoupdating the date when i put the off hire date in, can anyone help me. Another thing i want is for the spreadsheet to flag up "overdue" every 30 days when payment column has no value, this will allow me to keep track of overdue hire payments not yet received and which are overdue.
Thanks