You can always change the date and time of your training sessions.
When changing the training date, the weather information, zone settings and effort are recalculated accordingly.

Change date on an uploaded workout

Hover over the date in the training detail view and click on the date.
Select a new date, and then click on "Update training" to apply the changes.

Change date for a manually created workout

For a manual or planned workout, you can change the date in the corresponding fields at any time.

Change date by drag&drop in the training calendar

Another and easy way to change the date is to move the activity within the calendar. This works in the weekly and monthly view of the training calendar.