Lecture 37

Dates and times

MCS 260 Fall 2021
Emily 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), but officially it can be different in each Python installation.

• 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.)

Time

The main thing I think the time module is good for is measuring the elapsed time between two events, e.g.

        import time

        t0 = time.time()
        for x in huge_list:
            complicated_function(x)
        t1 = time.time()

        print("That took {:.2f} seconds".format(t1-t0))
    

Keep in mind this measures "wall clock" time, not the total CPU time spent doing actual work.

datetime

Module includes class datetime.datetime for representing a Gregorian calendar time as 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)
• datetime.datetime(2021,8,23,10,3,27) — object representing 27 seconds after 10:03am on Aug 23, 2021

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.

Time zone handling

Everything we've covered so far uses naive datetime objects, not labeled by a specific time zone.

Real world applications typically need to account for time zones (and their complexity).

We won't cover this in MCS 260 but I want to suggest:

• If you need to work with time zones in Python, know that built-in support for this is limited.
• There are a number of add-on modules that can make time zone handling easier.

String to datetime

The datetime module can take a string and convert it to a datetime object, which is one of its most powerful features.

• datetime.datetime.strptime(date_string,format) — Convert a string to a datetime, assuming it uses the format described in format (%-codes indicate datetime parts).
• datetime.datetime.strptime("2021-12-03","%Y-%m-%d") — Parse a year-month-day string and make a datetime object out of it.

Format codes for strptime include (see full list):

• %Y = year
• %m = month (two digit)
• %B = full month name
• %d = day (two digit)
• %A = weekday name (e.g. Friday)
• %a = weekday abbreviation (e.g. Fri)
• %H = hour (two digit, 24 hour format)
• %I = hour (two digit, 12 hour format)
• %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 ⇆ 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

Comparison

For datetime objects, the comparison operator < means "is earlier in time than".

        datetime.datetime(1999,11,19) < datetime.datetime.now() # True!
    

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.

Recommendations

• For past events, store timestamp or UTC datetime
• Convert to a datetime object when displaying
• For future events, it's really complicated! (e.g. what if time zone rules change between now and then?)

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.)

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.

Not required in MCS 260.