People
An employee record is a person as the company relates to them: their role, their department, when they started, and their absence.
Employees
An employee belongs to an organization and is usually linked to a user account — usually, not always. Somebody can exist in the records without having a login.
An employee record is personal data
It falls under the platform's GDPR machinery: it is part of a data subject's export, and it is anonymised on erasure. That includes people who never had an account.
Absence
Holiday, sickness, and whatever other kinds you define. Requests are approved by whoever your configuration says, and approved absence appears in the calendar the rest of the team plans against.
Calendar
Absence, deadlines, deliveries and anything else with a date can be shown in one calendar — because a calendar is a listing display, not a separate agenda.
Time
Time recorded against projects is the same time an employee worked. Nothing is entered twice.