Change Date Inside a Docker Container

1. Overview

Docker containers are a lightweight way to package, manage, distribute, and deploy applications. Each container runs in its own isolated environment, with its own file structure, network, and processes. It’s very handy to run containers on any system since they are portable. Sometimes, we need to change the date and time inside a Docker container.

In this tutorial, we’ll learn how to change the date and time of a Docker container.

2. Importance of Changing the Date Inside a Container

Changing the date inside a container has multiple benefits. It helps test application behavior and debug time-related bugs. It also allows testing time-dependent features and ensures compliance and auditing requirements.

We can observe how an application behaves when the system clock is ahead or behind the expected time by altering the date inside the container. In addition, it is useful to test if an application is ready for deployment in different environments, time periods, and scenarios.

3. Using the alpine-libfaketime Package

We can use the alpine-libfaketime package to set the time and date inside a Docker container. Also, it helps in manipulating the system clock for a specific process. The alpine-libfaketime library uses the time-wrapping technique to change the perception of time for a process without actually modifying the system clock. As a result, it is safer and more flexible than directly changing the system clock.

All instructions to install alpine-libfaketime can be put into a Dockerfile. To demonstrate, let’s check out the Dockerfile to update the date:

FROM groovy:alpine
COPY --from=trajano/alpine-libfaketime /faketime.so /lib/faketime.so
ENV LD_PRELOAD=/lib/faketime.so DONT_FAKE_MONOTONIC=1

The above Dockerfile loads alpine-libfaketime into the container’s processes using the LD_PRELOAD environment variable when alpine-libfaketime is installed. This allows for the manipulation or alteration of the default system clock.

Furthermore, we can use the faketime command to set the date and time for a specific process or group of processes. Here’s the command to build the image:

$ docker build -f fakedemo-java.Dockerfile . -t fakedemo

This will build the image successfully. Let’s look into the command to run the above image:

$ docker run --rm -e FAKETIME=+15d fakedemo groovy -e "print new Date();"
WARNING: Using incubator modules: jdk.incubator.vector, jdk.incubator.foreign
Mon Mar 13 18:35:06 GMT 2023

In the output of the above command, we can see that the date of the container is increased by 15 days. This way, developers can easily update the container’s time for testing and development purposes.

4. Using the tzdata Package

The tzdata package provides timezone information for various regions around the world. Furthermore, we can use it to update the timezone of a Docker container.

To demonstrate, let’s create a Dockerfile to install tzdata library:

FROM ubuntu:latest
RUN apt-get update \
&&  apt-get install -y tzdata \
&&  ln -fs /usr/share/zoneinfo/America/New_York /etc/localtime \
&&  dpkg-reconfigure --frontend noninteractive tzdata
CMD ["date"]

In the above Dockerfile, we installed the tzdata package and created a symbolic link between the timezone file for the desired region and the /etc/localtime file inside the container. Then, using the dpkg-reconfigure command, we updated the timezone. Finally, the default timezone of the docker container will update to /America/New_York timezone.

Let’s look at the command to build the image:

$ docker build -t timezone .

To run the container using the timezone image, we need to use the following command:

$ docker run --rm  --name timezone timezone
Sun Feb 26 13:38:40 EST 2023

In the above output, we can see that the container’s timezone has been changed to EST from its default UTC timezone.

5. Conclusion

In this article, we explored various ways to change the date and time of a Docker container.

Changing the date and time inside a Docker container can be useful for testing applications that rely on the system clock for time-sensitive operations. First, we examined the importance of changing the date and time. After that, we updated the time with the alpine-libfaketime and tzdata library.