Configure Time Machine after reinstalling Mac OS to take previous backups into account I have reinstalled my Mac OS (10.9) and had to specify again my external hard drive for backups in Time Machine preferences.  Now i can enter Time Machine and see my previous backups.

However, Time Machine preferences show

Oldest backup: None
Latest backup: None


In particular, the estimated size for a new backup indicates that it will have to make a complete backup, without taking into account that most files in my home directory have not changed since the previous backup (before the Mac OS reinstallation).

Can i configure Time Machine to recognize the previous backups and to only backup modified files in my home directory?
Yes. The command line utility can assist

man tmutil


You'll want to use the verbs inheritbackup, setdestination and/or associatedisk to point out the pre-existing backup you wish to adopt by the new Mac/OS. 

This is covered in very nice detail on the help / manual page - pay attention to the section after this sentence:


  A concrete example of when and how you would use associatedisk: