user2827286 user2827286 - 2 months ago 6
Python Question

Converting and comparing 2 datetimes

I need help trying to convert a string to datetime, then comparing it to see if it is less than 3 days old. I have tried both with the time class, as well as the datetime class, but I keep getting the same error:

TypeError: unsupported operand type(s) for -: 'str' and 'str'


here is the code I have tried:

def time_calculation():
time1 = "2:00 PM 5 Oct 2016"
time2 = "2:00 PM 4 Oct 2016"
time3 = "2:00 PM 1 Oct 2016"
timeNow = time.strftime("%Y%m%d-%H%M%S")
#newtime1 = time.strftime("%Y%m%d-%H%M%S", time.strptime(time1, "%I:%M %p %d %b %Y"))
newtime1 = datetime.strptime(time1, "%I:%M %p %d %b %Y").strftime("%Y%m%d-%H%M%S")
print("the new time1 is {}".format(newtime1))
#newtime2 = time.strftime("%Y%m%d-%H%M%S", time.strptime(time2, "%I:%M %p %d %b %Y"))
newtime2 = datetime.strptime(time2, "%I:%M %p %d %b %Y").strftime("%Y%m%d-%H%M%S")
print("the new time2 is {}".format(newtime2))
#newtime3 = time.strftime("%Y%m%d-%H%M%S", time.strptime(time3, "%I:%M %p %d %b %Y"))
newtime3 = datetime.strptime(time3, "%I:%M %p %d %b %Y").strftime("%Y%m%d-%H%M%S")
print("the new time3 is {}".format(newtime3))
timeList = []
timeList.append(newtime1)
timeList.append(newtime2)
timeList.append(newtime3)

for ele in timeList:
deltaTime = ele - timeNow
if deltaTime.days < 4:
print("This time was less than 4 days old {}\n".format(ele.strftime("%Y%m%d-%H%M%S")))


the commented out parts are what I did with time, while the others are with datetime.

the error happens at the line where I try to compare the current time with each time in the list, but it takes them as strings instead of datetimes and won't subtract them so I can compare them. (In the for loop at the bottom.)

Answer

Indeed, don't convert back to strings and instead work with datetime objects. As noted in the error message str - str isn't an operation that's defined (what does it mean to subtract a string from another?):

"s" - "s" # TypeError

Instead, initialize timeNow with datetime.now(), datetime instances support subtracting. As a second suggestion, subtract ele from timeNow and not timeNow from ele:

def time_calculation():
    # snipped for brevity
    timeNow = datetime.now()

    # snipped

    for ele in timeList:
        deltaTime = timeNow - ele
        if deltaTime.days < 4:
            print("This time was less than 4 days old {}\n".format(ele.strftime("%Y%m%d-%H%M%S")))

Prints out:

time_calculation()
the new time1 is 2016-10-05 14:00:00
the new time2 is 2016-10-04 14:00:00
the new time3 is 2016-10-01 14:00:00
This time was less than 4 days old 20161005-140000

This time was less than 4 days old 20161004-140000

Which I'm guessing is what you were after.