WEBVTT 1 00:00:00.360 --> 00:00:01.790 Hi guys, and welcome back. 2 00:00:01.790 --> 00:00:03.500 In this video, we're going to talk about 3 00:00:03.500 --> 00:00:07.970 how to do Date arithmetic with the timedelta class. 4 00:00:07.970 --> 00:00:10.230 Something that we see quite often is people trying 5 00:00:10.230 --> 00:00:11.980 to add dates together. 6 00:00:11.980 --> 00:00:13.300 Something like this. 7 00:00:13.300 --> 00:00:16.620 Datetime.datetime.now plus now again, 8 00:00:16.620 --> 00:00:18.310 is always gonna result in an error, 9 00:00:18.310 --> 00:00:20.750 because you can't really add two dates. 10 00:00:20.750 --> 00:00:22.160 What should the output be? 11 00:00:22.160 --> 00:00:24.820 For example, right now it's the year 2020. 12 00:00:24.820 --> 00:00:26.970 Right, if you add today to today, 13 00:00:26.970 --> 00:00:29.130 should we end up in the year 4040. 14 00:00:29.130 --> 00:00:31.820 So clearly, that doesn't make much sense. 15 00:00:31.820 --> 00:00:35.000 But what does make sense is trying to add 16 00:00:35.000 --> 00:00:38.480 a certain number of days, for example, to the current date, 17 00:00:38.480 --> 00:00:41.340 and to do that we use the timedelta class. 18 00:00:41.340 --> 00:00:43.610 So here I've created the current date 19 00:00:43.610 --> 00:00:45.950 using datetime.datetime.now, 20 00:00:45.950 --> 00:00:49.560 and then I've created a seven day difference object 21 00:00:49.560 --> 00:00:53.940 that is a datetime.timedelta at days equals seven. 22 00:00:53.940 --> 00:00:57.080 Now we can add that to a datetime object 23 00:00:57.080 --> 00:01:00.490 and it will increase the date it represents by seven days 24 00:01:00.490 --> 00:01:02.600 without affecting the time in this case, 25 00:01:02.600 --> 00:01:05.490 but we could also subtract it from the current date. 26 00:01:05.490 --> 00:01:07.510 And then we would go back by seven days 27 00:01:07.510 --> 00:01:11.210 timedelta also accepts other arguments as well as days. 28 00:01:11.210 --> 00:01:13.010 We've got seconds, microseconds, 29 00:01:13.010 --> 00:01:16.100 milliseconds, minutes, hours, and weeks. 30 00:01:16.100 --> 00:01:18.520 Some of these get converted to other units, 31 00:01:18.520 --> 00:01:21.210 the timedelta object only stores three of these, 32 00:01:21.210 --> 00:01:22.470 but don't worry about that. 33 00:01:22.470 --> 00:01:25.430 Essentially, it'll handle anything you throw at it 34 00:01:25.430 --> 00:01:27.460 within these arguments. 35 00:01:27.460 --> 00:01:31.055 Another common issue is to try to compare datetimes 36 00:01:31.055 --> 00:01:33.110 and timedelta objects. 37 00:01:33.110 --> 00:01:36.060 On the left we've got something that doesn't really work, 38 00:01:36.060 --> 00:01:37.460 we've got the current date, 39 00:01:37.460 --> 00:01:39.720 and then we've got a one week differential. 40 00:01:39.720 --> 00:01:42.800 If you try to compare one is greater than the other. 41 00:01:42.800 --> 00:01:44.120 That's not gonna make much sense 42 00:01:44.120 --> 00:01:48.130 because one of them represents a current point in time. 43 00:01:48.130 --> 00:01:50.660 The other one represents a difference in time, 44 00:01:50.660 --> 00:01:52.820 it's comparing apples to oranges. 45 00:01:52.820 --> 00:01:55.670 What does work though, is the example on the right, 46 00:01:55.670 --> 00:01:57.160 we've got the current date. 47 00:01:57.160 --> 00:01:58.810 Then we've got the date next week, 48 00:01:58.810 --> 00:02:00.930 which we've calculated by adding seven days 49 00:02:00.930 --> 00:02:02.120 to the current date, 50 00:02:02.120 --> 00:02:04.420 and you can check whether one is greater than the other, 51 00:02:04.420 --> 00:02:05.650 that's totally fine. 52 00:02:05.650 --> 00:02:08.230 So what I'm getting at is when you add a datetime 53 00:02:08.230 --> 00:02:11.760 to a timedelta, you end up with a datetime. 54 00:02:11.760 --> 00:02:13.810 Note that it is clearly not that useful 55 00:02:13.810 --> 00:02:16.630 today is always gonna be smaller than next week 56 00:02:16.630 --> 00:02:18.350 in terms of dates and times. 57 00:02:18.350 --> 00:02:20.840 But what does make sense is for us to try to do 58 00:02:20.840 --> 00:02:22.290 something like this. 59 00:02:22.290 --> 00:02:24.910 Here, we're asking the user for a date. 60 00:02:24.910 --> 00:02:27.280 Then we're parsing it using strptime 61 00:02:27.280 --> 00:02:29.320 as we learned in the last video, 62 00:02:29.320 --> 00:02:33.030 and then we are calculating whether the date 63 00:02:33.030 --> 00:02:35.960 that they entered is greater than next week. 64 00:02:35.960 --> 00:02:38.143 So here we've got their datetime. 65 00:02:39.150 --> 00:02:40.540 Here we've got next week, 66 00:02:40.540 --> 00:02:43.410 we should calculate by adding seven days to today. 67 00:02:43.410 --> 00:02:45.010 And then we compare them 68 00:02:45.010 --> 00:02:46.510 and if they're not painting within a week, 69 00:02:46.510 --> 00:02:48.060 then we call them a slacker. 70 00:02:48.060 --> 00:02:50.030 I should run this programme for myself as well. 71 00:02:50.030 --> 00:02:52.010 I need to paint my shed. 72 00:02:52.010 --> 00:02:54.180 So yeah, that's everything that we've got here 73 00:02:54.180 --> 00:02:57.980 for comparing datetimes and timedeltas and using timedeltas 74 00:02:57.980 --> 00:03:00.210 to perform datetime arithmetic. 75 00:03:00.210 --> 00:03:01.400 I hope you guys have learned something. 76 00:03:01.400 --> 00:03:04.350 Thank you for watching, and I'll see you in the next video.