Datetime notebook: ATM350, Spring 2026

Meteorological data is inherently time-based. We will have frequent need to parse, assign, and manipulate date and time strings/objects in our data analysis and visualization workflows. We used the `date` command in our exploration of the `bash` shell earlier in the course. Python has its own set of date and time tools that are extremely powerful, once you get the hang of it!¶

The `datetime` module includes the following classes:¶

date
datetime
time
timedelta
timezone
tzinfo

It also includes these associated methods:¶

strftime
strptime

Imports¶

We’ll import some of these various classes as follows, as well as one from a separate Python library that handles time zones:

from datetime import datetime, timedelta
from zoneinfo import ZoneInfo

Get the current time by making a call to datetime’s datetime.now method. It produces a datetime object with the year, month, day, hour, minute, second and microsecond.¶

current = datetime.now()

current

datetime.datetime(2026, 3, 13, 0, 23, 51, 222293)

print(current)

2026-03-13 00:23:51.222293

tz_utc = ZoneInfo("UTC")
tz_est = ZoneInfo("EST")
tz_eastern = ZoneInfo("America/New_York")

Re-generate the current datetime object, this time passing in the optional argument tz to make it timezone-aware.

current = datetime.now(tz=tz_utc)

current

datetime.datetime(2026, 3, 13, 0, 23, 51, 374023, tzinfo=zoneinfo.ZoneInfo(key='UTC'))

This datetime object contains a variety of attributes that we can list, as well as reformat.¶

current.year

2026

current.month

3

We can use the strftime method on a datetime object in order to produce date/time strings in a specified format.¶

We’ll play around with a few of these options in the cells below, but visit https://docs.python.org/3/library/datetime.html#strftime-strptime-behavior for full documentation of strftime!¶

# %A or %a will list the full name of the day of the week:
current.strftime("%A")

'Friday'

# %y will list the two-digit year, %m, the two-digit month, and %d, the two-digit date:
print(current.strftime("%y"))
print(current.strftime("%m"))
print(current.strftime("%d"))
print(current.strftime("%y%m%d"))

Let’s combine the year, month, and day, into a date string we’re used to using in ATM 350! We can define a new object called current_str as the output from our `strftime` method, still using our the datetime object `current` we created using the datetime.now() method:¶

current_str = current.strftime("%Y is the year, %m is the month, and %d is the day, and Ross is the man")
print(current_str)

2026 is the year, 03 is the month, and 13 is the day, and Ross is the man

Let’s incorporate the two-digit hour into the string representation:

current.strftime("%y%m%d%H")

'26031300'

What if we wanted to define a more human-friendly string to use; say, in the title of a graphic?

current.strftime("%y%m%d at %HZ")

'260313 at 00Z'

# Enter your code here:
current.strftime("%d %B %Y at %H%M %Z")

'13 March 2026 at 0023 UTC'

Click to reveal only AFTER you have tried your own code!

current.strftime("%d %B %Y at %H%M %Z")

We can also create datetime objects with any date/time/timezone we want.¶

date_first = datetime(2024,5,14,17,tzinfo=tz_eastern)

date_second = datetime(2024,5,14,16,tzinfo=tz_est)

date_third = datetime(2024,5,15, 0, tzinfo=tz_utc)

date_first

datetime.datetime(2024, 5, 14, 17, 0, tzinfo=zoneinfo.ZoneInfo(key='America/New_York'))

print(date_first.year)
print(date_first.month)
print(date_first.hour)
print(date_first.minute)
print(date_first.second)
print(date_first.tzinfo)

2024
5
17
0
0
America/New_York

We can also use some basic arithmetic to calculate time between dates!¶

dt1 = date_first - date_second
print(f"dt1 = {dt1}")

dt2 = date_third - date_second
print(f"dt2 = {dt2}")

dt1 = 0:00:00
dt2 = 3:00:00

Datetime’s `timedelta` class allows us to easily compute dates and times earlier or later than a base time.¶

Note that these objects are defined as a period of time, rather than a date.¶

t1 = timedelta(weeks = 2, days = 5, hours = 1, seconds = 33)
t2 = timedelta(days = 4, hours = 11, minutes = 4, seconds = 54)
t3 = t1 + t2

print("t3 =", t3)

t3 = 23 days, 12:05:27

t1 = timedelta(seconds = 33)
t2 = timedelta(seconds = 54)
t3 = t1 - t2

print("t3 =", t3)
print("t3 =", abs(t3))

t3 = -1 day, 23:59:39
t3 = 0:00:21

Timedelta includes a total_seconds() method:¶

t = timedelta(days = 6, hours = 13, seconds = 53, microseconds = 232436)
print(f"total seconds = {t.total_seconds()}" )
print(f"total minutes = {t.total_seconds()/60}")

total seconds = 565253.232436
total minutes = 9420.887207266667

Note that `months` is not a valid timedelta argument!! Why not?¶

t1 = timedelta (months = 1)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[23], line 1
----> 1 t1 = timedelta (months = 1)

TypeError: __new__() got an unexpected keyword argument 'months'

However, if our `datetime` object includes a month, we can compute a time delta in months via the `relativedelta` function from a separate package, `dateutil`:¶

from dateutil.relativedelta import relativedelta

current + relativedelta(months=2)

datetime.datetime(2026, 5, 13, 0, 23, 51, 374023, tzinfo=zoneinfo.ZoneInfo(key='UTC'))

As we learned at the beginning of this notebook, datetime’s datetime class includes the strftime method which takes a datetime object and converts it into a string, which can be formatted. Review from earlier...¶

# current date and time
curtime = datetime.now(tz=tz_utc)

string1 = curtime.strftime("%H:%M:%S")
print("string1:", string1)

string2 = curtime.strftime("%m/%d/%Y, %H:%M:%S")
# mm/dd/YY H:M:S format
print("string2:", string2)

string3 = curtime.strftime("%d/%m/%Y, %H:%M:%S")
# dd/mm/YY H:M:S format
print("string3:", string3)

string1: 00:23:52
string2: 03/13/2026, 00:23:52
string3: 13/03/2026, 00:23:52

Conversely, the strptime method takes an appropriately-formatted date/time string and converts it into a datetime object.¶

#Start with a string:
date_string = "10 June, 2023"
print("date_string =", date_string)

#Convert to a date object, making sure to include the appropriate formatting we used in the string:
date_object = datetime.strptime(date_string, "%d %B, %Y")
print("date_object =", date_object)

date_string = 10 June, 2023
date_object = 2023-06-10 00:00:00

date_string_2026 = "2026-02-24T00:00:00 UTC"
date_object_2026 = datetime.strptime(date_string_2026, "%Y-%m-%dT%H:%M:%S %Z")
date_object_2026

datetime.datetime(2026, 2, 24, 0, 0)

date_object_2026.tzinfo

We can assign it the proper timezone:

date_object_2026 = date_object_2026.replace(tzinfo=tz_utc)
date_object_2026

datetime.datetime(2026, 2, 24, 0, 0, tzinfo=zoneinfo.ZoneInfo(key='UTC'))

Calculate a new date and convert it into its string representation.¶

new_date_object = date_object_2026 + timedelta (days=14)
print(new_date_object)
new_date_string = new_date_object.strftime ("%d %B, %Y %Z")
print(new_date_string)

2026-03-10 00:00:00+00:00
10 March, 2026 UTC

# Enter your code here: