Unless your phone was jailbroken, "hacking" of the device itself is extremely unlikely. A compromised account is more likely, but I'm not picturing how an alarm could be set on your phone without somebody actually having the device in an unlocked state in their hands.
Read this document if you think your Apple ID has been compromised. --> If you think your Apple ID has been compromised - Apple Support
For iOS 15 or earlier, if you want to see if anyone else has access to your device or accounts, click here --> See who has access to your iPhone or iPad - Apple Support
For iOS 16 or later see how Safety Check on iPhone works to keep you safe. How Safety Check on iPhone works to keep you safe - Apple Support
Related materials:
Personal Safety User Guide
Personal Safety User Guide - Apple Support
> open the Table of Contents and review the articles
A document with general information about security and your Apple ID --> Security and your Apple ID - Apple Support
Contact Apple for help with Apple ID account security by clicking on this link --> Contact Apple for help with Apple ID account security - Apple Support "This article provides country-specific Apple Support contact information for customers seeking help with their Apple ID password or other security-related issues."
Are you sure you didn't just set the alarm to that time by mistake?