I am getting Time is not intialised error

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Error message:

   File "/home/angshuman/catkin_ws/src/barc/src/state_estimation_KinBkMdl_sim.py", line 81, in <module>
    state_estimation()
  File "/home/angshuman/catkin_ws/src/barc/src/state_estimation_KinBkMdl_sim.py", line 56, in state_estimation
    rate        = rospy.Rate(loop_rate)
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rospy/timer.py", line 59, in __init__
    self.last_time = rospy.rostime.get_rostime()
  File "/opt/ros/indigo/lib/python2.7/dist-packages/rospy/rostime.py", line 209, in get_rostime
    raise rospy.exceptions.ROSInitException("time is not initialized. Have you called init_node()?")
rospy.exceptions.ROSInitException: time is not initialized. Have you called init_node()?

My code:

#!/usr/bin/env python

# ---------------------------------------------------------------------------
# Licensing Information: You are free to use or extend these projects for
# education or reserach purposes provided that (1) you retain this notice
# and (2) you provide clear attribution to UC Berkeley, including a link
# to http://barc-project.com
#
# Attibution Information: The barc project ROS code-base was developed
# at UC Berkeley in the Model Predictive Control (MPC) lab by Jon Gonzales
# (jon.gonzales@berkeley.edu). The cloud services integation with ROS was developed
# by Kiet Lam  (kiet.lam@berkeley.edu). The web-server app Dator was
# based on an open source project by Bruce Wootton
#
# The original code is being modified by Angshuman Goswami(agoswam@clemson.edu)
# from Clemson University working in Efficient Mobility via Connectivity
# and Control (EMC2) lab
# ---------------------------------------------------------------------------

import rospy
import time
import os
from sensor_msgs.msg import Imu
from barc.msg import ECU, Encoder, Z_KinBkMdl
from numpy import pi, cos, sin, eye, array, zeros, unwrap
from observers import kinematicLuembergerObserver, ekf
from system_models import f_KinBkMdl, h_KinBkMdl
from tf import transformations
from numpy import unwrap

# input variables [default values]
d_f      = 0         # steering angle [deg]
acc      = 0         # acceleration [m/s]


# ecu command update
def ecu_callback(data):
    global acc, d_f
    acc         = data.motor        # input acceleration
    d_f         = data.servo        # input steering angle

# state estimation node
def state_estimation_sim():

    # initialize node
    rospy.init_node('state_estimation_sim', anonymous=True)
    # topic subscriptions / publications
    rospy.Subscriber('ecu', ECU, ecu_callback)
    state_pub   = rospy.Publisher('state_estimate_sim', Z_KinBkMdl, queue_size = 10)

    # get vehicle dimension parameters
    L_a = 0.125       # distance from CoG to front axel
    L_b = 0.125       # distance from CoG to rear axel
    vhMdl   = (L_a, L_b)

    # set node rate
    loop_rate   = 50
    dt          = 1.0 / loop_rate
    rate        = rospy.Rate(loop_rate)
    t0          = time.time()


    while not rospy.is_shutdown():

        # publish state estimate
        (x, y, psi, v) = z

        # publish information
        state_pub.publish( Z_KinBkMdl(x, y, psi, v) )

        # collect measurements, inputs, system properties
        # collect inputs
        u   = array([ d_f, acc ])
        args = (u,vhMdl,dt)

        # apply EKF and get each state estimate
        z= f_KinBkMdl

        # wait
        rate.sleep()

if __name__ == '__main__':
    try:
       state_estimation_sim()
    except rospy.ROSInterruptException:
        pass