That's correct from my experience. For example, enable the phone at 10am, you're 24 hour timer begins. Put your phone into Airplane mode at 9:59am next morning ( wouldn't cut it that close IRL) and then enable it again at 6pm, 24 timer begins again.
The Ts & Cs of the day pass indicate it's only when you actively
use your AT&T account while abroad. Still hoping for some confirmation, though, from an FTer.
Use and assessment of fee: Once you add this option to your device, the
first time you use data or voice, or send a text message in an International Day Pass (IDP) country, you’ll automatically be charged a $10 daily fee. The fee covers additional data, voice, or text messages sent for 24-hours from the initial use in any IDP country during that period. You’ll be charged an additional $10 fee for all subsequent 24-hour periods in which you use data or voice, or send a text message on a device set up with International Day Pass in an IDP country until you cancel the service.
Incidental data usage: Apps on your device, continue to use data even when you may not be aware of the usage. Such incidental data usage may trigger the $10 daily fee. To avoid unintended use of International Day Pass, turn off the roaming feature for your smartphone or device.
https://www.att.com/offers/internati.../day-pass.html