import arcpy, datetime
import os

mxd = arcpy.mapping.MapDocument(r"D:\Workings\PTV\GTFS_Vimeo.mxd")
df = arcpy.mapping.ListDataFrames(mxd, "Layers")[0]
df.time.currentTime = df.time.startTime

while df.time.currentTime <= df.time.endTime:
    theTime = df.time.currentTime.strftime('%HH_%MM')
    print "theTime = " + theTime
    fileName = theTime + ".png"
    print "This is fileName: " + fileName
    arcpy.mapping.ExportToPNG(mxd, os.path.join(r"D:\Workings\PTV\Output", fileName))
    df.time.currentTime = df.time.currentTime + df.time.timeStepInterval
del mxd