How can I set an alarm on all my Android devices at once for free? I would like to be able to set an alarm on all my Android devices at once.

The application Timely Alarm Clock has this feature ("Cloud synchronization enables a seamless experience across phones and tablets.") but the latter costs $2.5. 

Are there any free alternatives?
I try to not read this as "is there an app...", as that would be off-topic according to our guidelines (see What topics can I ask about here? for details).

So apart from looking for a different app to save some peanuts, you could live with the ads (according to its description, the app is available for free with ads; there's no note to the cloud feature being restricted to payment, neither does the related description on the product's webpage, nor the FAQ on How can I synchronize my devices mention that).
according to Krampus' comment below, the cloud functionality seems to be restricted to "premium users" indeed, though none of the material cited says so

Alternatively, there is the possibility to utilize a synchronized calendar (like the Google Calendar) for that: Create an event, which then is synchronized across your devices. So the alert should trigger on all of them.