Clockify is a straightforward time tracker and timesheet that allows you to get your job done productively. It lets you and your team track the working hours across various projects. With its timer, you could track how long the assistants are working. Under timekeeping capability, other features include tracking hour using a timer, categorizing time by project, and marking time as billable.
What makes this an efficient tool for executive assistants is that it is able to breakdown the time spend visually. Through customizable reports, you could see how long you have been working. You could even use Clockify for team activities, allowing you to set hourly rates, see who worked on what, and others.