user2827286 user2827286 - 1 year ago 44
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 Source

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.