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Hi,

Time in ROS have a standard representation : http://wiki.ros.org/roscpp/Overview/Time

You can test is with some simple command :

roscore&
rostopic pub /topic_name geometry_msgs/PoseStamped "header: auto
pose:
  position:
    x: 0.0
    y: 0.0
    z: 0.0
  orientation:
    x: 0.0
    y: 0.0
    z: 0.0
    w: 0.0"  -r 1 &
rostopic echo --offset /topic_name/header

The result of those commands will be :

seq: 6
stamp:
  secs: -71
  nsecs: 997840166
frame_id: ''
---
seq: 7
stamp:
  secs: -72
  nsecs: 998944997
frame_id: ''
---
seq: 8
stamp:
  secs: -73
  nsecs: 998456955
frame_id: ''
---

There is indeed 1 second of offset between messages (1 second because of the -r 1 in rostopic pub), displayed as a negative value because the message stamp is behind the current timestamp. If you fill the stamp with 'now' value every 1 second, the duration will be -1 secs everytime.