rospy.get_time() returns 0 when using gazebo

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Hi, When I run my code, I run into two problems. First, when I call rospy.get_time(), it returns 0. Secondly, in my while loop, perhaps due to the first issue, it only iterates once. Here is my code:

 #!/usr/bin/env python
import rospy
from geometry_msgs.msg import Twist

topicName = "cmd_vel_mux/teleop"

pub = rospy.Publisher("cmd_vel_mux/input/teleop", Twist, queue_size=10)
rospy.init_node('drive_time')




b_time = rospy.get_time()
print b_time
n = 0
while rospy.get_time() < b_time + 5:
    n+=1
    t = Twist()
    t.linear.x = 0.1
    pub.publish(t)
    rospy.sleep(0.1)
print n


print "done"
t = Twist()
t.linear.x = 0.1
pub.publish(t)

I am running a turtlebot in gazebo using turtlebot_world.launch. I tried changing the param use_sim_time to false and to true and it didn't work either way. When I initialize a node by typing it into the interpreter, rospy.get_time() works fine. Any ideas as to why this is happening?