Create a new event or task as you normally would

Go to the date/time option in that event or task editor and set the date and time as you would like it to apply to your default.

Before you save that setting, tap the menu icon at the top right and choose the 'set default' option. You will get a popup confirmation of the new default setting you applied. If its not what you expected, change the date/time and then choose the set default option again. You can do this as many times as needed until the default is exactly as you want.

Then when you create a new event or task, the default you set will automatically be applied.