How to correctly fake time?

Question

In order to resolve a specific timezone related bug on a software project that I'm working on, I'm trying to replicate a change on the system's clock using timedatectl to check my software behavior.

I'm trying to achieve that by running

timedatectl set-timezone America/New_York
timedatectl set-local-rtc 1
timedatectl set-ntp false
timedatectl set-time "2017-03-12 01:58:50" && hwclock -w

When I do that and run timedatectlright after, I get the expected following time:

Local time: Sun 2017-03-12 01:58:51 EST
Universal time: Sun 2017-03-12 06:58:51 UTC
RTC time: Sun 2017-03-12 01:58:51
Time zone: America/New_York (EST, -0500)
System clock synchronized: no
systemd-timesyncd.service active: no
RTC in local TZ: yes

However, 10 seconds after this, when the minute changes (to xx:59), the local and universal time kind of "reboot" to the current time

Local time: Wed 2019-08-07 21:01:41 EDT
Universal time: Thu 2019-08-08 01:01:41 UTC
RTC time: Sun 2017-03-12 01:58:57
Time zone: America/New_York (EDT, -0400)
System clock synchronized: no
systemd-timesyncd.service active: no
RTC in local TZ: yes

What am I missing here?

My setup is an Vagrant Ubuntu 18 VM (Linux vagrant 4.15.0-51 / vm box "bento/ubuntu-18.04").

Answer 1

This won’t explain the behaviour you’re seeing, but it should answer the question in the title.

Instead of changing the system’s time, you could use a tool such as faketime and the TZ environment variable:

$ date; faketime -f -15d date; TZ=America/New_York faketime -f -15d date
Thu  8 Aug 10:21:13 CEST 2019
Wed 24 Jul 10:21:13 CEST 2019
Wed 24 Jul 04:21:13 EDT 2019

This will allow you to run your program with its own time zone, date and time, without affecting the rest of the system.

Note that as it uses a $LD_PRELOAD hack to inject code in applications, it won't work for statically linked executables or setuid/setgid executables.