How to call a service with begin and end time as request/input

edit

Hi,

I am looking to call laser_assembler call back service -AssembleScans .

time begin
time end
---
sensor_msgs/PointCloud cloud
  std_msgs/Header header
    uint32 seq
    time stamp
    string frame_id
  geometry_msgs/Point32[] points
    float32 x
    float32 y
    float32 z
  sensor_msgs/ChannelFloat32[] channels
    string name
    float32[] values

I am looking to call this service in my velocity call back function but not how to input choose the start and end time.

There is an example in laser_assembler package using timer call back

void timerCallback(const ros::TimerEvent& e)
{
    AssembleScans srv;
    srv.request.begin = e.last_real;
    srv.request.end   = e.current_real;
    client_.call(srv)
}

Now I am looking the same service from a velocity call back

void  velocity_cb (const geometry_msgs::Twist& cmd_vel)
{
    AssembleScans srv;
    srv.request.begin = ???;
    srv.request.end   = ???;
}

Not sure how to input srv.request.begin , srv.request.end .