Comments on ROS Time

"ROS has builtin time and duration primitive types, which roslib provides as the ros::Time and ros::Duration classes, respectively. A Time is a specific moment (e.g. "today at 5pm") whereas a Duration is a period of time (e.g. "5 hours"). Durations can be negative."

The function Time::now(), which is used in most of the codes I am working with uses the system time:

Time Time::now()
{
if (!use_system_time_)
{
boost::mutex::scoped_lock lock(g_sim_time_mutex);
Time t = sim_time_;
return t;
}

Time t;
getWallTime(t.sec, t.nsec);

return t;
}

When I create the map in "real time" the time used is the one from my computer:

"When running with a real robot, the ROS client libraries will use "wall-clock" time (i.e. the time your computer reports). "

In the case of the playback of the bag files I have noticed the following:

"if you are playing back sensor data into your system, you may wish to have your time correspond to the timestamps of the sensor data. "

Concerning Clock server:

"If you are playing back a bag file with rosbag play, using the --clock option will run a Clock Server while the bag file is being played."