As I recall :utc_datetime stores timezone information where :naive_datetime does not and stores things as utc always. I think… ^.^;
Regardless, datetimes in the future you want to store with timezone information. Timezones change all the time and if someone plans something on, say, March 18th at 5pm, and daylight savings starts on March 20th but then that gets changed to start on March 12th then if you stored the datetime as utc in the database then you are off by an hour where if you stored it with the proper timezone then it will still be correct.
Past times are of course fine to store timezoneless.
If you are sure your visitors will always send a timezone parameter you might as well do the transormation in the browser and always assume that your backend receives UTC date/times. Or you can transform the date/time to UTC inside your users’ browsers if a timezone parameter is not always present.