All milestones appear as 01/01/1970
In my installation milestones are saved as unix time (seconds), not microseconds. The time (unix time) is divided by 1000000, and it appears as 1970.
I do not know if it has anything to do with my installation; but my there shouldn't be much special with my installation.
I use trac 0.11.1 combined with the zipped tracjsganttplugin (0.4) found on the wiki page.
Python: 2.6.2 (release26-maint, Apr 19 2009, 02:11:59) [GCC 4.3.3]