how to calculate timedelta python

Question

What I am trying to do is to subtract 7 hours from a date. I searched stack overflow and found the answer on how to do it here. I then went to go read the documentation on timedelta because I was unable to understand what that line in the accepted answer does, rewritten here for ease:

from datetime import datetime

dt = datetime.strptime( date, '%Y-%m-%d %H:%M' )
dt_plus_25 = dt + datetime.timedelta( 0, 2*60*60 + 30*60 )

Unfortunately, even after reading the documentation I still do not understand how that line works.

What is the timedelta line doing? How does it work?

Additionally, before I found this stackoverflow post, I was working with time.struct_time tuples. I had a variable tm:

tm  = time.strptime(...)

I was simply accessing the hour through tm.tm_hour and subtracting seven from it but this, for obvious reasons, does not work. This is why I am now trying to use datetime. tm now has the value

tm = datetime.strptime(...)

I'm assuming using datetime is the best way to subtract seven hours?

Note: subtracting seven hours because I want to go from UTC to US/Pacific timezone. Is there a built-in way to do this?

Answer 1

timedelta() generates an object representing an amount of time—the Greek letter delta is used in math to represent "difference". So to compute an addition or a subtraction of an amount of time, you take the starting time and add the change, or delta, that you want.

The specific call you've quoted is for generating the timedelta for 2.5 hours. The first parameter is days, and the second is seconds, so you have (0 days, 2.5 hours), and 2.5 hours in seconds is (2 hours * 60 minutes/hour * 60 seconds/minute) + (30 minutes * 60 seconds / minute).

For your case, you have a negative time delta of 0 days, 7 hours, so you'd write:

timedelta(0, -7 * 60 * 60) 

... or timedelta(0, -7 * 3600) or whatever makes it clear to you what you're doing.

Answer 2

What is the timedelta line doing? How does it work?

It creates a timedelta object.

There are two meanings of "time".

• "Point in Time" (i.e, date or datetime)

• "Duration" or interval or "time delta"

A time delta is an interval, a duration, a span of time. You provided 3 values.

• 0 days.

• 2*60*60 + 30*60 seconds.

Answer 3

Note: subtracting seven hours because I want to go from UTC to US/Pacific timezone. Is there a built-in way to do this?

Yes there is: datetime has built-in timezone conversion capabilities. If you get your datetime object using something like this:

tm = datetime.strptime(date_string, '%Y-%m-%d %H:%M')

it will not have any particular timezone "attached" to it at first, but you can give it a timezone using

tm_utc = tm.replace(tzinfo=pytz.UTC)

Then you can convert it to US/Pacific with

tm_pacific = tm_utc.astimezone(pytz.all_timezones('US/Pacific'))

I'd suggest doing this instead of subtracting seven hours manually because it makes it clear that you're keeping the actual time the same, just converting it to a different timezone, whereas if you manually subtracted seven hours, it looks more like you're actually trying to get a time seven hours in the past. Besides, the timezone conversion properly handles oddities like daylight savings time.

To do this you will need to install the pytz package, which is not included in the Python standard library.