Open Schedule and click the add button, then New Job/Task. Full steps below.
Where: Schedule > add button > New Job/Task, or a project's Schedule tab
Plans: Pro, Agency and Enterprise (or an active trial)
Steps
Open Schedule and click the add button, then New Job/Task.
Fill in the title and dates.
Turn off the all-day option to reveal the Start Time and End Time fields, and set them.
Save. Changing the date later keeps the times you entered.
Rules and limits
The Schedule page is included from Pro upward. Starter and Free plans see an upgrade prompt instead of the calendar.
Still stuck?
If a step does not match what you see on screen, stop there and open a ticket from Support > My Tickets with a screenshot of that step. The team will look at the account directly.