Dates, times, random numbers and maths

Loan due dates, receipts, countdowns, games and reports all need dates and numbers. Python's standard library covers them.

datetime basics

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from datetime import datetime, date, timedelta

now = datetime.now()
today = date.today()
print("Now:", now)
print("Today:", today)
print(today.year, today.month, today.day)
print("Weekday (Monday=0):", today.weekday())

Formatting dates (strftime)

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from datetime import datetime

d = datetime(2026, 12, 12, 14, 30)          # Jamhuri Day, 2:30 pm
print(d.strftime("%d/%m/%Y"))                # 12/12/2026
print(d.strftime("%A, %d %B %Y"))            # Saturday, 12 December 2026
print(d.strftime("%I:%M %p"))                # 02:30 PM
print(d.strftime("%Y-%m-%d %H:%M"))          # 2026-12-12 14:30

Code	Meaning	Example
`%d`	Day	12
`%m`	Month number	12
`%B` / `%b`	Month name / short	December / Dec
`%Y`	Year	2026
`%A` / `%a`	Weekday / short	Saturday / Sat
`%H:%M`	24-hour time	14:30
`%I %p`	12-hour + AM/PM	02 PM

Reading dates from text (strptime)

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from datetime import datetime

text = "05/03/2026"
d = datetime.strptime(text, "%d/%m/%Y")
print(d.date(), d.strftime("%A"))

Date arithmetic with timedelta

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from datetime import date, timedelta

borrowed = date(2026, 9, 1)
due = borrowed + timedelta(days=30)
print("Loan due:", due)

today = date(2026, 9, 28)
left = (due - today).days
print("Days left:", left)

birthday = date(2000, 6, 15)
age_days = (today - birthday).days
print("Age in years (approx):", age_days // 365)

# every Monday for the next 4 weeks
start = today + timedelta(days=(7 - today.weekday()) % 7)
for i in range(4):
    print((start + timedelta(weeks=i)).strftime("%a %d %b"))

The math module

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import math

print(math.sqrt(144), math.pi)
print(math.ceil(4.1), math.floor(4.9))
print(math.pow(2, 10), 2 ** 10)
print(round(math.pi, 3))

# area of a round water tank base with radius 1.5 m
r = 1.5
print(f"Area: {math.pi * r ** 2:.2f} m²")

# how many 50-seat buses for 173 students?
print("Buses:", math.ceil(173 / 50))

random: games, samples and simulations

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import random

print(random.randint(1, 6))                     # dice: 1 to 6
print(random.random())                          # 0.0 to 1.0
print(random.choice(["Heads", "Tails"]))
names = ["Amina", "Brian", "Chebet", "Dennis", "Esther"]
print(random.sample(names, 2))                  # 2 different winners
random.shuffle(names)
print(names)

random.seed(42)                                 # same "random" results every run (for testing)
print(random.randint(1, 100), random.randint(1, 100))

random is fine for games. For passwords, PINs and security codes use the secrets module, which is designed to be unpredictable:

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import secrets, string
otp = "".join(secrets.choice(string.digits) for _ in range(6))
print("Your code:", otp)
print(secrets.token_urlsafe(16))

statistics

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import statistics as st

marks = [45, 78, 92, 60, 78, 55]
print("Mean:", round(st.mean(marks), 1))
print("Median:", st.median(marks))
print("Mode:", st.mode(marks))
print("Std dev:", round(st.stdev(marks), 1))

Mini project: a loan schedule

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from datetime import date, timedelta

principal = 30000
monthly_rate = 0.015            # 1.5% a month
months = 4
payment = round(principal * (1 + monthly_rate * months) / months, 2)

due = date(2026, 10, 5)
print(f"{'#':<3}{'Due date':<14}{'Payment':>10}")
for n in range(1, months + 1):
    print(f"{n:<3}{due.strftime('%d %b %Y'):<14}{payment:>10,.2f}")
    due += timedelta(days=30)
print(f"Total repaid: KSh {payment * months:,.2f}")

Who uses dates and numbers in code?

Where	Example
Lending apps and SACCOs	Due dates, late penalties, interest schedules
Schools	Term dates, attendance by week, age on admission
Shops and pharmacies	Expiry dates, daily sales reports, opening hours
HR and payroll	Working days in a month, leave balances, overtime
Games and quizzes	Random questions, dice, shuffled answers
Data analysis	Averages, medians and spread of marks, sales or survey answers

Working with times and durations

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from datetime import datetime, timedelta

clock_in = datetime(2026, 9, 28, 7, 55)
clock_out = datetime(2026, 9, 28, 17, 20)
worked = clock_out - clock_in
hours = worked.total_seconds() / 3600
print("Worked:", worked, f"= {hours:.2f} hours")

overtime = max(0, hours - 8)
print(f"Overtime: {overtime:.2f} h, pay KSh {overtime * 250:,.0f}")

meeting = datetime(2026, 10, 1, 9, 0)
reminder = meeting - timedelta(minutes=30)
print("Send reminder at", reminder.strftime("%H:%M on %d %b"))

timedelta supports days, seconds, minutes, hours and weeks. There is no months= because months have different lengths.

Adding months safely

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from datetime import date
import calendar

def add_months(d, n):
    month = d.month - 1 + n
    year = d.year + month // 12
    month = month % 12 + 1
    last_day = calendar.monthrange(year, month)[1]
    return date(year, month, min(d.day, last_day))

start = date(2026, 1, 31)
for i in range(1, 5):
    print(add_months(start, i))      # Feb 28, Mar 31, Apr 30, May 31
print("Days in Feb 2028:", calendar.monthrange(2028, 2)[1])
print("2028 leap year?", calendar.isleap(2028))

Comparing and sorting dates

Dates compare like numbers, so you can sort them, find the earliest and check ranges:

Python · runs live in the interactive lesson

from datetime import date

expiry = {
    "Panadol": date(2026, 11, 30),
    "Amoxil": date(2026, 10, 10),
    "ORS": date(2027, 3, 1),
}
today = date(2026, 10, 1)
for drug, exp in sorted(expiry.items(), key=lambda kv: kv[1]):
    days = (exp - today).days
    status = "EXPIRED" if days < 0 else "expires soon" if days <= 30 else "ok"
    print(f"{drug:<8} {exp}  {days:>4} days  {status}")

Working days

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from datetime import date, timedelta

holidays = {date(2026, 10, 10), date(2026, 10, 20)}   # Mazingira Day, Mashujaa Day (example list)

def working_days(start, end):
    d, count = start, 0
    while d <= end:
        if d.weekday() < 5 and d not in holidays:
            count += 1
        d += timedelta(days=1)
    return count

print("Working days in Oct 2026:", working_days(date(2026, 10, 1), date(2026, 10, 31)))

Keep a list of public holidays in your program or a settings file and update it each year, because some holiday dates move or are gazetted at short notice.

ISO format: the safest way to store dates

Store dates as YYYY-MM-DD (ISO 8601) in files and databases. They sort correctly as text and every system understands them:

Python · runs live in the interactive lesson

from datetime import date, datetime

d = date(2026, 3, 5)
print(d.isoformat())                         # 2026-03-05
print(date.fromisoformat("2026-12-25"))
print(datetime.fromisoformat("2026-12-25T18:30:00"))
print(sorted(["05/03/2026", "12/01/2026"]))   # wrong order as text!
print(sorted(["2026-03-05", "2026-01-12"]))   # correct order

Time zones in brief

datetime.now() gives the computer's local time with no time zone attached ("naive"). Servers often run on UTC, three hours behind Kenya (EAT, UTC+3). For anything shared between systems, use aware datetimes:

Python · runs live in the interactive lesson

from datetime import datetime, timezone, timedelta

EAT = timezone(timedelta(hours=3), "EAT")
utc_now = datetime(2026, 9, 28, 6, 0, tzinfo=timezone.utc)
print("UTC:", utc_now)
print("Nairobi:", utc_now.astimezone(EAT))

In larger programs, zoneinfo.ZoneInfo("Africa/Nairobi") provides the official zone.

Money and decimals

Floats store numbers in binary, so some decimals can't be represented exactly. For money, either round carefully or use Decimal:

Python · runs live in the interactive lesson

from decimal import Decimal, ROUND_HALF_UP

print(0.1 + 0.2)                                  # 0.30000000000000004
print(Decimal("0.1") + Decimal("0.2"))            # 0.3

price = Decimal("1999.995")
print(price.quantize(Decimal("0.01"), rounding=ROUND_HALF_UP))
vat = (Decimal("2500") * Decimal("0.16")).quantize(Decimal("0.01"))
print("VAT:", vat)

Another common approach is to store amounts in cents as integers (KSh 12.50 → 1250) and only format them for display.

Compound interest and loans

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principal = 50000
annual_rate = 0.12
years = 3

simple = principal * annual_rate * years
compound = principal * (1 + annual_rate / 12) ** (12 * years) - principal
print(f"Simple interest:   KSh {simple:,.2f}")
print(f"Compound (monthly): KSh {compound:,.2f}")

# reducing-balance monthly payment (standard loan formula)
r = annual_rate / 12
n = years * 12
payment = principal * r / (1 - (1 + r) ** -n)
print(f"Monthly payment: KSh {payment:,.2f}, total KSh {payment * n:,.2f}")

A random simulation: estimating chances

Simulations repeat a random experiment many times to estimate a probability:

Python · runs live in the interactive lesson

import random

random.seed(1)
trials = 10000
both_six = sum(1 for _ in range(trials)
               if random.randint(1, 6) == 6 and random.randint(1, 6) == 6)
print(f"Estimated chance of double six: {both_six / trials:.3f} (exact {1/36:.3f})")

# weighted choice: 70% chance of 'small prize'
prizes = random.choices(["small", "medium", "big"], weights=[70, 25, 5], k=10)
print(prizes)

Common mistakes

Mistake	Problem	Fix
`datetime.strptime("5/3/2026", "%m/%d/%Y")`	Day and month swapped (US order)	Kenya uses day/month: `%d/%m/%Y`
Storing dates as `"05/03/2026"` text	Sorts wrongly	Store ISO `2026-03-05`
`timedelta(months=1)`	TypeError	Use an add-months function
Using `random` for OTPs	Predictable	Use `secrets`
Comparing money floats with `==`	Rounding errors	Round, use Decimal or integer cents

Practice

Ask for a date of birth as text (DD/MM/YYYY) and print the exact age in years.
Print a 6-month repayment schedule using add_months instead of +30 days.
Simulate 1,000 coin tosses and print how many heads and tails.
List all Fridays in December 2026.
Given a list of sale timestamps, count sales per hour of the day.

Think about it: Why might adding timedelta(days=30) each month give the wrong due dates for a loan that is due on the 5th of every month?Show answer

Months have 28 to 31 days, so adding 30 days drifts: 5 Jan + 30 days is 4 Feb, then 6 Mar, and so on. Use a month-aware calculation that keeps the same day number (and clamps to the last day for the 29th to 31st).

Check yourself

Which datetime method formats a date as text?
Show answer
strftime
Which class adds or subtracts days from a date?
Show answer
timedelta
What does math.ceil(4.1) give?
Show answer
5
Which random function picks a whole number between two values?
Show answer
randint
Which module should you use for security codes and passwords?
Show answer
secrets
Which timedelta method converts a duration to a number of seconds?
Show answer
total_seconds
Which date format sorts correctly as text and should be used for storage? (name or pattern)
Show answer
ISO
Which module gives exact decimal arithmetic for money?
Show answer
decimal
Which calendar function tells you how many days are in a month?
Show answer
monthrange

Exercise

Use timedelta to print the date 30 days after date(2026, 9, 1). The output should be 2026-10-01.

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Lesson 16 of 20 in Python · Printable course notes