import arcpy
mxd = arcpy.mapping.MapDocument(r"C:\Project\Asthma.mxd")
df = arcpy.mapping.ListDataFrames(mxd, "ER Visits by Residence Lat/Lon")[0]
df.time.currentTime = df.time.startTime
while df.time.currentTime <= df.time.endTime:
    #An example str(newTime) would be: "2008-12-29 02:19:59"
    #The following line splits the string at the space and takes the first
    #item in the resulting string.
    fileName = str(df.time.currentTime).split(" ")[0] + ".png"
    arcpy.mapping.ExportToPNG(mxd, r"C:\Project\Output\\" + fileName, df)
    df.time.currentTime = df.time.currentTime + df.time.timeStepInterval
del mxd