I see the same thing, and there are reports in these forums dating back a couple of years on the same issue. So what the "last backup date" is reporting I don't know but it doesn't seem to be Time Machine, or that function is broken. You can send this command to get the results of the last five backups:
syslog |grep successfully |tail -n 5
(thanks to Kiraly for the command)
will set that to run. on my desktop i run geektool and run this command
syslog -F '$Time $Message' -k Sender com.apple.backupd -k Time ge -72h | tail -n 10
it shows a bit more detail but perhaps too much
have now setup a scheduled unix task in ARD to run it on the other computers as well
one weird thing is that for 3 macs it reports the data, on the other 2 the status is success but it does not display any data, but when i check locally on the macs themselves i can see they have had succesfull backups. for those 3 where it shows the data the status is the last entry of the output (ie Ejected Time Machine network volume.)
and the detail
Mar 27 13:01:55 Found 118 files (9.7 MB) needing backup
Mar 27 13:01:55 1.95 GB required (including padding), 1.43 TB available
Mar 27 13:02:49 Copied 4159 files (18.3 MB) from volume Macintosh HD.
Mar 27 13:02:49 Created new backup: 2013-03-27-130249
Mar 27 13:03:00 Starting post-backup thinning
Mar 27 13:03:24 Deleted /Volumes/Time Machine Backups/Backups.backupdb/LL-MBA/2013-03-26-115902 (13.7 MB)
Mar 27 13:03:24 Post-back up thinning complete: 1 expired backups removed
Mar 27 13:03:24 Backup completed successfully.
Mar 27 13:03:28 Ejected Time Machine disk image: /Volumes/Data/LL-MBA.sparsebundle
Mar 27 13:03:28 Ejected Time Machine network volume.
Mar 27 08:10:55 Created new backup: 2013-03-27-081053
Mar 27 08:11:12 Starting post-backup thinning
Mar 27 08:11:12 No post-back up thinning needed: no expired backups exist
Mar 27 08:11:12 Backup completed successfully.
Mar 27 08:11:17 Ejected Time Machine disk image: /Volumes/Data/MCG-MBA.sparsebundle
Mar 27 08:11:17 Ejected Time Machine network volume.
Mar 27 13:43:06 Starting manual backup
Mar 27 13:43:06 Network destination already mounted at: /Volumes/Data
Mar 27 13:43:18 Disk image /Volumes/Data/MCG-MBA.sparsebundle mounted at: /Volumes/Time Machine Backups
Mar 27 13:43:18 Backing up to: /Volumes/Time Machine Backups/Backups.backupdb
Mar 27 07:34:34 Found 34 files (8 KB) needing backup
Mar 27 07:34:34 2.2 GB required (including padding), 1.43 TB available
Mar 27 07:35:39 Copied 693 files (4.4 MB) from volume Macintosh HD.
Mar 27 07:35:41 Created new backup: 2013-03-27-073539
Mar 27 07:35:43 Starting post-backup thinning
Mar 27 07:36:12 Deleted /Volumes/Time Machine Backups/Backups.backupdb/rjb-mba2/2013-03-26-043641 (25 MB)
Mar 27 07:36:12 Post-back up thinning complete: 1 expired backups removed
Mar 27 07:36:13 Backup completed successfully.
Mar 27 07:36:16 Ejected Time Machine disk image: /Volumes/Data-1/rjb-mba2.sparsebundle
Mar 27 07:36:17 Ejected Time Machine network volume.
it is better than not knowing anything about any of the macs. just waiting for the other macs to come back on our network and i will configure them as well