Working With Date & Time

Almost every real application deals with dates and times — birthdays, deadlines, log timestamps, scheduling. Python's datetime module provides a clean, timezone-aware API for parsing, formatting, computing differences, and scheduling events. In this lesson we'll explore the core classes date, time, datetime, timedelta, and the formatting mini-language.

• Import: from datetime import date, time, datetime, timedelta.
• Get current moment with datetime.now() or date.today().
• Format with strftime(fmt); parse with strptime(text, fmt).
• Compute differences with subtraction — you'll receive a timedelta.

from datetime import datetime, date, timedelta

today = date.today()                      # 2024-09-12
now   = datetime.now()                    # 2024-09-12 14:30:55.123456

later = now + timedelta(days=7, hours=3)  # one week and 3 hours later
diff  = later - now                       # timedelta(days=7, ...)

stamp = now.strftime("%Y-%m-%d %H:%M")    # "2024-09-12 14:30"
parsed = datetime.strptime("12/09/2024", "%d/%m/%Y")

• Core classes: date holds Year/Month/Day. time holds Hour/Minute/Second/Microsecond. datetime combines both. timedelta represents a duration.
• Getting the current time: datetime.now() returns the local current moment. datetime.utcnow() returns UTC (deprecated in 3.12 — use datetime.now(timezone.utc)).
• Formatting (strftime): Convert a datetime to a string with a format spec: %Y year, %m month, %d day, %H/%M/%S hour/minute/second, %A weekday name, etc.
• Parsing (strptime): Reverse of strftime: pass a string and a matching format string to get a datetime object.
• Date arithmetic: Add or subtract timedelta from a date or datetime to shift it. Subtracting two datetimes returns a timedelta.
• Timezones: Use zoneinfo.ZoneInfo (Python 3.9+) to attach timezone info: datetime.now(ZoneInfo('Asia/Kolkata')).

from datetime import datetime
now = datetime.now()
print(now)
print(now.year, now.month, now.day)

from datetime import datetime
now = datetime.now()
print(now.strftime("%d-%b-%Y %I:%M %p"))

from datetime import datetime
d = datetime.strptime("15/08/1947", "%d/%m/%Y")
print(d.strftime("%A, %B %d, %Y"))

from datetime import date, timedelta
today = date.today()
in_100 = today + timedelta(days=100)
print('100 days from today:', in_100)

from datetime import date
birth = date(2000, 5, 21)
today = date.today()
print("Age in days:", (today - birth).days)

from datetime import datetime
from zoneinfo import ZoneInfo
ist = datetime.now(ZoneInfo("Asia/Kolkata"))
utc = datetime.now(ZoneInfo("UTC"))
print(ist.strftime("%H:%M %Z"))
print(utc.strftime("%H:%M %Z"))

• Prefer datetime objects over plain strings — type safety wins.
• Store everything in UTC; convert to local time only when displaying.
• Use ISO-8601 (YYYY-MM-DDTHH:MM:SS+00:00) for serialised dates.
• Use strptime with strict formats — never trust user-typed dates.
• For complex parsing, use the dateutil third-party library.
• Beware DST jumps — always use zoneinfo when working with local timezones.

• Mixing naive and aware datetimes: raises TypeError when subtracting.
• Using datetime.utcnow(): produces a naive UTC time; prefer datetime.now(timezone.utc).
• Wrong format string: strptime raises if any character mismatches.
• Treating dates as strings: string comparisons may pass but math will fail.
• Hard-coding month lengths: use timedelta(days=1) + the calendar module.
• Forgetting timezone in scheduled jobs: servers often run in UTC, not local time.

• Problem 1: Print today's date in the format Monday, 12 September 2024.
• Problem 2: Ask the user for their birth date and print their age in years.
• Problem 3: Compute the number of days between two user-entered dates.
• Problem 4: Print the date that falls 90 days from today.
• Problem 5: Read a list of timestamps and group them by month using a dict.
• Problem 6: Build a small reminder script that prints how many days are left until New Year.

• Q1. What's the difference between date, time, and datetime?
• Q2. What is a timedelta? Show an example.
• Q3. Why do we prefer UTC for storage?
• Q4. How do you parse a date from a string?
• Q5. What is a 'naive' datetime vs an 'aware' datetime?
• Q6. Why is datetime.utcnow() being deprecated?

• Q1: How do I get the current date and time?
  datetime.now() returns the local current date and time. For UTC, use datetime.now(timezone.utc).
• Q2: How do I add or subtract days?
  Use timedelta(days=n). Add to or subtract from a date/datetime object.
• Q3: How do I format a date for the user?
  Use strftime with format specifiers like %Y, %m, %d. f-strings work too: f'{dt:%d-%b-%Y}'.
• Q4: How do I parse a date string?
  Use strptime(date_string, format). It is strict — a mismatch raises ValueError.
• Q5: How do I handle timezones?
  Use zoneinfo.ZoneInfo (Python 3.9+) — pass an IANA name like 'Asia/Kolkata' to attach timezone info.
• Q6: Where do I find format codes?
  See the official strftime cheat-sheet at docs.python.org — %Y, %m, %d, %H, %M, %S are the most common.