Creating a program that records speed in a speed trap

[astonavfc]

That is my code so far it says my subtraction is wrong can you point out anything I can do better.

Thanks.

from datetime import datetime
#Function to convert date from string to dateTime
def dateTimeIn(timestamp):
    dateTimeIn = datetime.strptime(strDateIn, "%d/%m/%Y %H:%M%S")
    return dateTimeIn
def dateTimeOut(timestamp):
    dateTimeOut = datetime.strptime(strDateOut, "%d/%m/%Y %H:%M%S")
    return dateTimeOut   

#Main Program   
myFileIn=open("CameraIn.txt","rt")
myFileOutput=open("Output.txt","rt")
for lineIn in myFileIn:
    lineIn=myFileIn.readline()
    theListIn=lineIn.split(",")
    regIn=theListIn[0]
    strTimeIn=theListIn[1]
    myFileOut=open("CameraOut.txt","rt")
    for lineOut in myFileOut:
        lineOut=myFileOut.readline()
        theListOut=lineOut.split(",")
        regOut=theListOut[0]
        strTimeOut=theListOut[1]
        if regIn==regOut:
            dateTimeIn-dateTimeOut