Is there any alternative to get the server date time other than Flox.getTime()?
I don't think there is. When the device is offline, all you've got is the system time ("new Date()"). In most cases, this should be rather accurate, since the system typically syncs with a time server. But you can't guarantee it. And when you can't reach Flox, it's unlikely that you can reach any other server. (Flox really has a good uptime.)
... oh, but if you mean "while the device is online", then just read on. 😉
I just want to make sure today is really today and not the user's system time! so that i can query some yesterday data from entities!
Here's one thing you can do: when you're online, occasionally store the difference between server time and local time. That's not as expensive as you might think, because you might be able to use "Entity.updatedAt" instead. (Whenever you create / modify an entity, simply trigger the logic saving the time offset, using that entity property as the current server time.)
Then, when you are offline, you can guess that the offset is still roughly the same. This information can be used to correct any system time information.