EasyCompilers

Vocabulary for this course

The terms the chapters assume, in plain words. You can open this from any chapter.

argument
A value you put inside the parentheses when you call a function, which the function then receives.
block
A group of lines that belong together under one opening line, such as the body of an if or a loop.
branch
A path a program can take when a condition sends it one way or another.
call site
The place in the code where a function is called, as opposed to where it is defined.
class
A template that bundles data with the code that works on it; each copy has its own values.
dictionary
A collection of key and value pairs, where a key is used to look up its value.
exception
An error that stops the normal flow of a program unless some code catches it.
function
A named piece of code that does one job and can be called whenever you need it.
function object
A function treated as a value, so it can be stored in a variable, passed to another function, or returned.
indentation
The spaces at the start of a line that show which block it belongs to.
integer
A whole number with no decimal point, such as 3, 0 or -12.
list
An ordered collection of values that you can grow, shrink and reach into by position.
parameter
A name in a function definition that stands for the value the caller will supply.
return value
The result a function hands back to the code that called it.
scope
The part of a program where a name can be used; a name made inside a function is not visible outside it.
traceback
The report Python prints when a program fails, listing the calls that led to the error.
variable
A name that refers to a value, so you can use that value again later or change it.

Python from the ground up

Dates and times

A value for a day: today, thirty days from now, and the two ways to write a date — for people and for machines.

Part 14 of 15

Intermediate14 min readBack to Courses

A module for time

Dates are a kind of value Python keeps in the standard library. from datetime import date brings the type in, and date.today() gives today as a date object — not text, but a value you can compare and do arithmetic with.

That difference matters. Text like "2026-10-07" cannot be added to anything; a date can be given thirty days, subtracted from another date, and printed in whichever format the reader needs.

Today, and thirty days later

from datetime import date, timedelta

today = date.today()

print(today)
print(today + timedelta(days=30))
print(today.strftime("%A, %d %B %Y"))

Two ways to write a date

The plain print shows the ISO format: 2026-10-07. Year first, then month, then day — the one ordering that sorts correctly as text, and the one machines and databases expect.

strftime writes a date for a person, using letters that stand for its parts: %A is the weekday name, %d the day of the month, %B the month name. "%A, %d %B %Y" becomes "Wednesday, 07 October 2026".

Counting days

Subtract one date from another and you get a timedelta — a length of time, with .days for the whole days it contains. It is the answer to questions a program is asked constantly: how long since the last order, how many days until the deadline.

timedelta(days=30) works the other way round: add it to a date and you have the date thirty days later, with the month lengths and leap years handled for you.

Today, in words and in thirty days

Print today in words — the weekday, the day and the month — and then print the date exactly thirty days from now.

Output
Click "Run" to execute your code

A date and a moment are different

date holds a day; datetime holds a day and a time; timedelta holds a length. Mixing them is a common source of TypeError, because a day cannot be compared with a moment without deciding which part of the day is meant. Pick the type that matches the question and stay with it.

A due date is a date. A log entry is a datetime. A reminder that fires at 09:00 needs a time as well, and the code that stores it should say so rather than appending a time to text.

Time zones are a separate problem

A naive datetime has no zone, so it means whatever the reader assumes. Two machines in different places can disagree about when it happened. Python keeps zone-aware values in the same module, and the rule is to convert at the edge — when reading input or showing output — and keep one consistent zone inside.

For a course program, a fixed date with no zone is fine. For anything that records when something happened, store UTC and convert for display; the arithmetic then stays correct across daylight-saving changes, which are exactly the days a hand-rolled offset gets wrong.

Store and compare dates in ISO form; change to words only when printing. A date written as "07/10/2026" is ambiguous across countries, and that ambiguity reaches production more often than anyone expects.

A date is a value, not a label

date(2026, 10, 7) builds a date from three numbers, and date.fromisoformat("2026-10-07") reads one from text. Once built, dates compare with < and > and subtract from each other, which text cannot do reliably.

Adding a timedelta gives a new date and leaves the old one alone, the same way adding a number to an integer gives a new integer. That is why deadline = start + timedelta(days=30) needs the result stored: the original start does not move.

Dates compare directly because they are values of the same kind: start < deadline is true or false without converting either to text. That makes a due-date check a comparison, not a string exercise.

Formatting and parsing are opposites

strftime turns a date into text; datetime.strptime, from the same module, turns text into a datetime. They take the same percent letters, and confusing them is a common afternoon.

Parsing needs the text to match the pattern exactly, including separators. datetime.strptime("07/10/2026", "%d/%m/%Y") reads day first; the same text with %m/%d/%Y reads July 10. The pattern is the only thing that decides.

Three date mistakes: ordering a date against text raises TypeError; strftime writes a date out while datetime.strptime reads text in, and they are easy to swap; and a date built from numbers is unambiguous, while "07/10/2026" means different days in different countries.

Try these next

  • Build date(2026, 1, 1) and print it in ISO form and in words.
  • Subtract two dates and print .days; then add the same number of days back to the earlier one.
  • Parse "2026-12-25" with fromisoformat and print the weekday with %A.

Days until the deadline

Two fixed dates are ready. Print how many days lie between them and the deadline in words. Then add 30 days to the deadline and print the new date. Do not use today’s date — the answer should be the same on every run.

Output
Click "Run" to execute your code

Check what you learned

One question at a time. A wrong answer explains itself — take another go.

Part 1 of 5

What is a timedelta?