Time on the job, in quarter hours
Log hours against a job in 15-minute increments, mark them billable or not, and turn billable time into an invoice line.
Every time entry belongs to a job. Enter the date, the hours in quarter-hour steps, whether the time is billable, and a note. Billable hours carry the rate they were worked at, so an invoice raised months later bills what was agreed at the time, not whatever the rate is now.
Billable time flows onto the job’s invoice as line items; once invoiced, the entry is locked. Time that should never be billed — a callback, a warranty visit — is recorded as non-billable so the job’s true cost still counts it.