Difference in movement between the input value and the actual movement

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-Robot: waffle Pi -SBC: Raspberry Pi 3

OS installed in SBC: Raspbian OS installed in Remote PC: Ubuntu 16.04 LTS (Xenial Xerus) ROS version: 1.12.14

Hello everyone, I wrote two codes to derive my robot one is for linear movement and the other is for the rotational movement. the problem is that the error is always increased by increasing the traveled distance (rotation /angle). That means if I enter 360 degrees rotation angle the robot will rotate almost 340 degrees (20 degrees error) and this error will higher when rotating for example 720 degrees and so on. When I apply the code using "fake node" there is no errors. Does anyone meet the same problem and have ideas to solve it? Best wishes Below is my code (for the rotation movement):

#!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist
#PI = 3.1415926535897   the basic position 

def move():  
        print("Let's rotate your robot")
        speed = input("Input your speed (degrees/sec):")
        angle = input("Type your distance (degrees):")
        clockwise = input("Clockwise?: ") 

        PI = 3.1415926535897
        angular_speed = speed*2*PI/360
        relative_angle = angle*2*PI/360


        #vel_msg.linear.x=0
        #vel_msg.linear.y=0
        #vel_msg.linear.z=0
        #vel_msg.angular.x = 0
        #vel_msg.angular.y = 0


        if clockwise:
            vel_msg.angular.z = -abs(angular_speed)
        else:
            vel_msg.angular.z = abs(angular_speed)

        t0 = rospy.Time.now().to_sec()
        #rospy.spin()  WRONG USE
        r=rospy.Rate(100)
        current_angle = 0
        while(current_angle < relative_angle):
            velocity_publisher.publish(vel_msg)
            t1 = rospy.Time.now().to_sec()
            current_angle = angular_speed*(t1-t0)
            r.sleep()



        vel_msg.angular.z = 0
        velocity_publisher.publish(vel_msg)
        #rospy.spin()   . if we uncomment tje rospy.spin the program will not stop automatically but only when pressing Ctrl+C

if __name__ == '__main__':
     try:
    rospy.init_node('move', anonymous=True)
        velocity_publisher = rospy.Publisher('/cmd_vel', Twist, queue_size=10)
        vel_msg = Twist()

        move()
     except rospy.ROSInterruptException:
           pass