no match for 'operator-' (operand types are 'ros::Time' and 'ros::Time')

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Hello,

I am trying to publish odometry following this tutorial from my Arduino but I have the following error:

Firmware_odometry.ino: In function 'void loop()':

Firmware_odometry:458:38: error: no match for 'operator-' (operand types are 'ros::Time' and 'ros::Time')

     dt = (current_time - last_time).toSec();

Here are a few lines of my firmware:

#include <ros.h>
#include <ros/time.h>
#include <tf/transform_broadcaster.h>
#include <nav_msgs/Odometry.h>
ros::NodeHandle  nh;
ros::Time current_time, last_time;

geometry_msgs::TransformStamped odom_ts;
geometry_msgs::Quaternion odom_quat; 
tf::TransformBroadcaster odom_bro;
nav_msgs::Odometry odom_msg;

ros::Publisher pub_odom("/odometry",&odom_msg);

long odom_time;
double  xo, yo, tho = 0;
double vx,vth = 0.1;
double vy = -0.1;
double dt, delta_xo, delta_yo, delta_tho;

void setup(){
 nh.initNode();
 nh.advertise(pub_odom);
}

void loop(){
// publish odometry information over ROS
  if(millis() >= odom_time){
  current_time = ros::Time::now();
  last_time = ros::Time::now();

  dt = (current_time - last_time).toSec();
  delta_xo = (vx * cos(tho) - vy * sin(tho)) * dt;
  delta_yo = (vx * sin(tho) + vy * cos(tho)) * dt;
  delta_tho = vth * dt;

  xo += delta_xo;
  yo += delta_yo;
  tho += delta_tho;

//publish the transform over tf
  odom_ts.header.stamp = current_time;
  odom_ts.header.frame_id = "odom";
  odom_ts.child_frame_id = "base_link";
  odom_ts.transform.translation.x = xo;
  odom_ts.transform.translation.y = yo;
  odom_ts.transform.translation.z = 0.0;
  odom_ts.transform.rotation = odom_quat;

//send the transform
  odom_bro.sendTransform(odom_ts);

//publish the odometry message over ROS
  odom_msg.header.stamp = current_time;
  odom_msg.header.frame_id = "odom";

//set the position 
  odom_msg.pose.pose.position.x = xo;
  odom_msg.pose.pose.position.y = yo;
  odom_msg.pose.pose.position.z = 0.0;
  odom_msg.pose.pose.orientation = odom_quat;

//set the velocity
  odom_msg.child_frame_id = "base_link";
  odom_msg.twist.twist.linear.x = vx;
  odom_msg.twist.twist.linear.y = vy;
  odom_msg.twist.twist.angular.z = vth;

  pub_odom.publish(&odom_msg);
  odom_time = millis()+10;
  }
  nh.spinOnce();
  delay(100);
}

Could you please guide me ?

Thanks in advance,

Shanika