There are a number of 'tricks' you can use to determine if the date time has changed, like saving the date time to local storage the first time the app is run. The next time it's run, check to see if the current date is later than the previously stored one and if so, update it. You could also have periodic checks throughout the game to see if it's changed.
If you're really concerned they may change it your best option is to get the time from a server using AJAX and compare it to the browser time.
Here's a quick project I slapped together to get the time from worldclockapi.com
+ System: On start of layout
-> AJAX: Request "http://worldclockapi.com/api/json/utc/now" (tag "time")
+ AJAX: On "time" completed
-> JSON: Parse JSON string AJAX.LastData
+ AJAX: On "time" error
-> (no actions)
+ JSON: Has key "currentDateTime"
-> Text: Set text to JSON.Get("currentDateTime")
It does get and display the proper time but, because it's a http and not a https, Chrome initially blocks the AJAX request. You have to click on the 'shield' to allow unsafe website requests. But, it shows you how it could be done.
Ultimately though, if they really, really want to cheat by changing the system time, they will. You just need to find ways to make it difficult.