Datetime Deep Dive: Parse, Format, Arithmetic

By the end of this lesson you can:

• Get the current moment with datetime.now() and pull it apart.
• Format a datetime into any string with strftime (and remember the codes).
• Parse a string back into a datetime with strptime.
• Add and subtract time with timedelta, and compare dates.

Look at how often automation touches time:

backup-2026-05-28.zip            timestamped filenames
"delete logs older than 30 days"  date arithmetic
"email the report every Monday"   day-of-week logic
"2026-05-28T14:30:00"            parsing API timestamps

Getting "now"

from datetime import datetime, date, timedelta

now = datetime.now()
print(now)               # 2026-05-28 14:30:12.345678
print(now.year, now.month, now.day)
print(now.hour, now.minute)
print(now.weekday())     # 0 = Monday … 6 = Sunday
print(date.today())      # just the date: 2026-05-28

Formatting: datetime → string (strftime)

now = datetime.now()

now.strftime("%Y-%m-%d")            # 2026-05-28   (great for filenames/sorting)
now.strftime("%Y%m%d-%H%M%S")       # 20260528-143012  (backup stamps)
now.strftime("%d/%m/%Y")            # 28/05/2026
now.strftime("%A, %d %B %Y")        # Thursday, 28 May 2026
now.strftime("%I:%M %p")            # 02:30 PM

The format codes you'll actually use:

%Y  4-digit year   2026      %H  hour 24h   14
%m  month 01-12    05        %M  minute     30
%d  day 01-31      28        %S  second     12
%B  month name     May       %A  weekday    Thursday
%b  month abbrev   May       %a  wkday abr  Thu
%p  AM/PM          PM        %I  hour 12h   02

Parsing: string → datetime (strptime)

# you give the EXACT format the string is in, and Python reads it
d = datetime.strptime("2026-05-28", "%Y-%m-%d")
d = datetime.strptime("28/05/2026 14:30", "%d/%m/%Y %H:%M")
print(d.year, d.hour)          # 2026 14

The trick: strptime's second argument is a recipe describing the input. If it doesn't match exactly, you get a ValueError — which is good, it catches malformed data. (For ISO strings like "2026-05-28T14:30:00", datetime.fromisoformat(s) is even easier.)

Arithmetic with timedelta

now = datetime.now()

tomorrow   = now + timedelta(days=1)
last_week  = now - timedelta(weeks=1)
in_2_hours = now + timedelta(hours=2)
deadline   = now + timedelta(days=30, hours=12)

# subtract two datetimes → a timedelta
age = now - datetime(2026, 1, 1)
print(age.days)                 # how many days into the year
print(age.total_seconds())      # the whole gap in seconds

timedelta represents a duration. Add it to a datetime to move forward/back; subtract two datetimes to get the gap between them.

Comparing dates

cutoff = datetime.now() - timedelta(days=30)
file_time = datetime.fromtimestamp(some_path.stat().st_mtime)

if file_time < cutoff:
    print("older than 30 days — a cleanup candidate")

Datetimes compare with <, >, == like numbers. This is exactly how "delete files older than N days" works — and it ties straight back to st_mtime from Lesson 6.

Goal: find log files older than a cutoff and report each one's exact age — the classic "retention policy" task, combining datetime with Lesson 6's file metadata.

from pathlib import Path
from datetime import datetime, timedelta

def old_logs(folder: str, days: int = 30) -> list[Path]:
    cutoff = datetime.now() - timedelta(days=days)
    old = []
    for log in Path(folder).rglob("*.log"):
        modified = datetime.fromtimestamp(log.stat().st_mtime)
        if modified < cutoff:
            age = datetime.now() - modified
            print(f"{log.name:30}  {age.days:4} days old  "
                  f"(modified {modified:%Y-%m-%d})")
            old.append(log)
    print(f"\n{len(old)} log(s) older than {days} days.")
    return old   # caller decides whether to delete — we only report

old_logs("logs", days=30)

app-2026-03-01.log              88 days old  (modified 2026-03-01)
app-2026-04-12.log              46 days old  (modified 2026-04-12)

2 log(s) older than 30 days.

Read the code

Three datetime tools work together: timedelta(days=days) builds the cutoff, fromtimestamp(st_mtime) turns the file's raw timestamp into a datetime, and the < comparison decides what's old. The {modified:%Y-%m-%d} inside the f-string is strftime in disguise — you can format dates right in the placeholder. Notice it returns the list rather than deleting: the script reports, a human (or Lesson 38's backup tool) decides.

datetime.now() gives the current moment as an object you can pull apart (.year, .weekday()). strftime formats a datetime into a string with codes like %Y-%m-%d %H:%M:%S — use big-to-small order in filenames so they sort chronologically. strptime parses a string back into a datetime given the exact format. timedelta represents durations: add it to move in time, or subtract two datetimes to measure the gap, and compare datetimes with </>. These power timestamped files, retention policies, and scheduling.