Something similar to this happened to me. I was messing around with the TM settings, without my (external) backup drive connected, and TM 'forgot' when the oldest and most recent backups were, just like in your picture. I had selected 'Change Disk...' by accident, but re-selected the disk that was already the backup disk.
I was worried that when I re-connected my backup drive TM was going to start a whole new backup, but it didn't. It backed up normally (a small, incremental backup), and the 'oldest' and 'latest' values were restored. Now, with the external backup drive not connected, I have dates in the 'oldest' and 'latest' fields, but the 'available' field is blank, which makes sense.
Do you know that TM has done a backup since you noticed the problem? What happens if you select 'backup now' -- does that bring the dates back?