One's days are numbered (8)
I believe the answer is:
calendar
'one's days are numbered' is the definition.
(as in dates given in a calendar)
This is all the clue.
(Other definitions for calendar that I've seen before include "Timetable" , "Day/month chart" , "Chart showing months and days" , "List of dates of land race" , "Table of days and dates" .)