Well, that's neither a bug, nor an interesting quirk.. The explanation is quite simple:
The Integer containing a julian date (jd) only stores the date, not the time. When you convert a unix-timestamp, which stores also the time, into a jd, the time-information won't be transferred into the jd. When converting back the jd into a unix-timestamp, you'll get a timestamp for the specific day at 2 o'clock am. ( 02:00:00 )
Sample:
Timestamp: 1121206013
JD: 2453565
Timestamp from JD: 1121212800
Original Timestamp "d.m.y H:i:s": 13.07.2005 00:06:53
Timstamp from JD "d.m.y H:i:s": 13.07.2005 02:00:00
greetz