5.1.1 Current Time

The Python time.time() function returns the same value as posixtime(datetime()) in MATLAB, a double-precision representation of current time in Unix epoch seconds, a continuous increment of seconds since midnight, January 1, 1970, ignoring leap seconds, to a resolution of a microsecond. Among other things, it is useful in Python for computing elapsed time of code segments.

5.1.2 Time String Formats

Format arguments for strftime() include all arguments accepted by the 1989 C standard library function of the same name plus a few others. In some cases, the output depends on the computer’s locale setting. This can be revealed by with the command locale in a terminal on Linux and macOS and with systeminfo in a command prompt on Windows (look for “System Locale” in the output). Examples in Table 5-1 use locale settings of en_US and de_DE, as taken from the Python project’s documentation.

5.1.3 tic, toc; %timeit

The difference of calls to time.time() is useful in a Python program, but when working interactively in ipython, it is easier to use its “magic” command %timeit to measure how long a command takes:

%timeit will run the command several times (seven in the preceding example) and return the mean time over all runs.

5.2.1 datetime Objects to and from Strings

Both datetime and time have a similarly named function, strptime(), that does the opposite: it takes a string containing a date and a second string describing the format, then returns a Python variable which can subsequently be used to perform date computations.

5.2.2 Time Deltas

5.3.1 UTC vs. Local Time

UTC, also called Zulu or Greenwich Mean Time, is the preferred timezone for recording events that span wide geographic areas. MATLAB and Python have several functions that explicitly work with UTC time as opposed to the local time on an individual computer.

Paradoxically, the datetime functions that return UTC time, datetime.utcnow() and datetime.utcfromtimestamp(), do not set tzinfo, and so they return timezone-unaware objects. In other words, the numeric values for day, hour, minute, and so on are correct, but there is no underlying metadata stating that this is the UTC timezone. If computational accuracy for subsequent date computations is important, the proper way to define UTC time is to use the functions without utc in their names and explicitly pass in tz=dateutil.tz.UTC [2]:

Similar conversions in MATLAB look like this:

5.4.1 Unix Epoch Seconds

Unix epoch seconds are independent of timezone, so the conversion from a timezone-enabled datetime object can be misleading:

For circular consistency, the Unix epoch value must be converted to the timezone where it was initially measured.

5.4.2 ISO 8601 Time String

Although the datetime module includes a .fromisoformat() function, curiously the function does not accept the trailing “Z” (for “Zulu,” also known as UTC); one must either omit the timezone designator or specify it as a numeric offset.

Also, since the ISO format function with a timezone offset explicitly sets the timezone, the local time variable is explicitly computed from the timezone-aware datetime object using the .astimezone() method :

5.4.3 Julian Date; Modified Julian Date; GPS Time

Anaconda comes with a module, astropy, which has powerful time conversion capabilities related to astronomy. These include Julian, Modified Julian, FITS, UT1, TAI, and GPS references.

The following examples use the US/Pacific timezone:

5.5.1 List Available Timezones

5.5.2 Date Increments Across Daylight Savings Transition

This example demonstrates correct handling of standard time to daylight savings time transition using rules for the Los Angeles entry in the timezone database: