By easy I mean easy to implement in code.
By reliable I mean that it doesn't depend sources who might fail under which there is no good control.
By secure I mean that it cannot be worked around or that if it could be worked around it would be pretty complicated.
The best solution I can think about for the moment is trying to check that the user has automatic date and hour and automatic time zone enabled. I thought this could be worked around by enabling flight mode, or maybe some other option, but at least in my smartphone, it returns to correct date. Guess there are some things about getting the date in phones I don't understand, anyway I can't be sure this works for all smartphones.
I know about other methods like using some NTP server, but I think this would add unnecesary complexity and less reliability.
I'd like to hear suggestions about which would be a good way to do this.
PS: I'm editing per request.
My app allows to make a certain action in the app only once per week.
It's not something that important what happens if the user manages to trick the app into making it believe that's another date, I don't really care much about maybe a 5% of users tricking the app to believe it's another date, but for this to happen at least all trivial ways in which the date retrieval could be worked around shouldn't be allowed.