Saving data with timestamps

Overview

In this lesson we are going to learn how to add a date and time to our data which is sometimes called a timestamp.
A timestamp records the current time that an event took place.
This becomes important later when we want to graph the data that we collected.
Computer settings are important with dates and times.
When we initially set up our Raspberry Pi, we need to define localisation options to be Melbourne, Australia.
The computer can then calculate our local time and even make allowances for daylight savings and atomic clock corrections.
It is also important that the Raspberry Pi is connected to the internet so that it can re-calibrate the onboard clock.
In python there is a special library called datetime that makes it easy to record and format dates and times.
In this lesson we will create a new program to get some experience with the datetime library before we add it back into our main program.

Learn how use the python datetime library.
Learn how to update our main program with the datetime library functions.
Learn how to verify that our data has saved correctly using the File Manager and Terminal.

In this example we will experiment with a new python library named datetime.
We will demonstrate how to get the current computer time, and also learn how to format the response.
More information on formatting the datetime output can also be obtained from W3Schools at the following URL: Python Dates (w3schools.com)

From the Raspberry Pi main menu drop down select Programming > Python3 (IDLE).

Enter the following code:
- import datetime – library import
- now = datetime.datetime(now) – creates a new datetime object named now
- print(now) – printing the now object
Save and Run the program

The output in the Shell should show the year, month, day, hour, minute, second and microsecond time components.

When we create a datetime object (now) we have access to a method for formatting output of the date object into more readable strings.
The method that does this is strftime().
To format the datetime object we need to use some special characters called Directives.
Each Directive has a percentage sign (%) and a single upper or lower-case letter (a-z, A-Z).
- %Y – full version of year – e.g. 2021
- %b – month name short version – e.g. Jan, Feb, Mar
- %d – day of the month 01 to 31 – e.g. 28
- %H – hour 00 to 24 – e.g. 08
- %M – minute 00 to 59 – e.g. 48
To use these Directives to format the date object add the following two lines of code.
- date_stamp = now.strftime(“%Y”)
- print(date_stamp)
The full code is included below.

Save and Run the program.
There are two lines of output:
- 2021-05026 03:45:43... – print out of the now instance of the datetime object
- 2021 – print out of the now datetime object formatted using the strftime() method to show the Year (%Y).

We can now keep on adding Directives to achieve our own customised format for the date and time.
In this example we have added the short version of the month using the (%b) Directive – e.g. Jan.
To add the full version of the month we would use the (%B) Directive – e.g. January.
To represent months as numbers 01 to 12 use the (%m) Directive – e.g. 01 (for January).