Congrats on the 710! You'll enjoy it for sure
I usually set mine at 0 when I start riding -- and then refer to the altitude figure as being relative to where you started. If you forget to do this, when you upload the exercises onto your computer, the PPP software can re-set the altitude data to start at zero, and also compensate for any 'drift' in the altitude readings.
Because of the way the altitude sensor works (measuring change in air pressure), it's very susceptible to temperature and weather variations. It does auto-compensate with temperature, but even still, if you calibrated it against "real" zero altitude, I wouldn't expect it to stay calibrated for more than a day.
You can reset the calibration to zero on the bike in two ways;
1/ turn the altimeter off, then back on, in the Options menu
2/ at the start of exercising (but not when it's active), get the altitude showing on the middle (large) display and hold down the top-left button for 5 seconds or so.