Lecture 37

Dates and times

MCS 260 Fall 2020
David Dumas

Reminders

Time

Python's time module can tell you the current timestamp, i.e. the time in second since a certain base point, the epoch. It can also do some other things.

The epoch is usually 0:00 on January 1, 1970 (GMT).

• time.time() — return current timestamp (float).
• time.gmtime(0) — return some data about the epoch for this Python installation.
• time.sleep(seconds) — pause execution for seconds seconds.

(The time module has many other functions.)

datetime

Primary module for working with dates and times. The main class is datetime.datetime representing a date and time (Gregorian calendar) broken into year, month, day, hour, minute, second, microsecond.

• datetime.datetime.now() — The current local time (as reported by the OS)
• datetime.datetime.utcnow() — The current time in UTC (equal to GMT)

These return "naive" datetimes; no time zone information is attached.

There are also datetime.date objects, representing dates in the Gregorian calendar, and datetime.time objects, representing a time of day.

These have similar behavior, so we will focus on datetime.datetime.

Datetime from string:

• datetime.datetime.strptime(date_string,format) — Convert a string to a datetime, assuming it uses the format described in format (%-codes indicate datetime parts).

Format codes include (see full list):

• %Y = year
• %m = month (two digit)
• %B = full month name
• %d = day (two digit)
• %H = hour (two digit, 24 hour)
• %I = hour(two digit, 12 hour)
• %M = minute (two digit)
• %S = second
• %p = AM/PM

Datetime to string:

If dt is a datetime object:

• dt.strftime(format) — converts dt to a string in the given format.

Datetime to/from timestamp:

If dt is a datetime object:

• datetime.datetime.fromtimestamp(ts) — Convert from a timestamp to a local date and time
• dt.timestamp() — Convert from datetime to a timestamp

Timedelta

Subtracting two datetime objects gives a datetime.timedelta object.

• datetime.timedelta(days=0, seconds=0, microseconds=0, milliseconds=0, minutes=0, hours=0, weeks=0) — build a new timedelta object
• delta.total_seconds() — convert an existing timedelta object to units of seconds

Internally, timedelta stores days, seconds, and microseconds. It supports division by other timedelta objects, and multiplication/division by numbers.

Time zone handling

Everything we've covered so far uses naive datetime objects, assuming the OS-reported local time zone when necessary.

Often, this isn't good enough.

But Python's built-in time zone handling is very limited. It can only represent a fixed offset from GMT.

pytz

The pytz module is one of the ways of working with time zones I recommend.

It is not in the standard library; install it with pip.

In pytz you build timezone objects and then localize naive datetimes to them (add time zone info), or normalize datetimes (convert from one zone to another).

• pytz.timezone(zone_name) — Build new time zone object representing a named zone like "US/Eastern"
• pytz.all_timezones — List of all recognized time zone names (>500)
• tzobj.localize(naive_dt) — Convert a naive datetime to the time zone represented by tzobj
• tzobj.normalize(dt)— Convert datetime that already contains time zone info to one in the time zone represented by tzobj

Recommendations

• For past events, store timestamp or UTC datetime
• Convert to user's preferred time zone when displaying
• For future events, it's complicated! (e.g. what if time zone rules change between now and then?)
  • Generally need to store the local time specification and the user's time zone.

dateutil

dateutil is another module not in the standard library that is often used for handling dates and times in Python.

(Ask pip to install python-dateutil.)

Like pytz it augments the functionality of datetime.

A nice feature of dateutil is that it has a function dateutil.parser.parse(s) to make a "best guess" at the meaning of a date string sof unknown format.