dbus-monitor displays double values to 5 significant digits, so you only lose 4 digits (10000 seconds = 2.5 hours).

[user@nokia ~]$ dbus-monitor --system
signal sender=:1.76 -> dest=(null destination) path=/org/gpsd; interface=org.gpsd; member=fix
double 1.20207e+09

[user@nokia ~]$ python
>>> from time import *
>>> ctime(1.20207e+09)
'Sun Feb 3 12:20:00 2008'
>>> ctime()
'Sat Feb 2 11:29:25 2008'

If you need exact timestamps, you can write your own signal listener (only a few lines of Python) or download the source of dbus-monitor and hack it to print doubles with %f instead of %g.