Stryd has an internal clock that get synchronized by the phone. The internal clock from Stryd continues to increment time, even when not connected to a phone or watch. However, if Stryd runs out of battery the clock stops. Certain unexpected events may cause this as well.
If Stryd ran out of battery, allow it to fully recharge. Once it has recovered, the clock will start ticking again but the time will be incorrect. When uploading Stryd data with the app on the phone, the activities will have an incorrect timestamp and may appear at the wrong date and time in the calendar.
To avoid this problem, keep your Stryd charged at all times. If you do end up with a depleted battery, connect to your Stryd with your phone to reset time and date. Performing a successful offline sync will reliably re-synchronize the clock. An alternative is to start an indoor run, allow the phone to connect to Stryd, count to 10, and cancel the run. Re-synchronizing the clock is recommended after any attempt to reset the device.