An Arbeitszeitkonto records differences between agreed working time and hours worked. The label is simple; the rules controlling the balance may be spread across the contract, a collective agreement and payroll documents.
Find the source of the rule
Mark every clause that refers to working time, overtime, a time account or a Tarifvertrag. A contract may not reproduce the detailed limits. Ask the agency which current agreement and provisions it applies and how you can access them.
Look beyond the balance
Useful questions include who can order extra hours, when credits become time off, whether a payout is possible, and what occurs when an assignment supplies fewer hours. Negative balances deserve particular care: do not assume from a payslip abbreviation alone why they arose.
Keep your own record
Save shift confirmations, agency statements and payslips. Record start, end and break times in a consistent format. The purpose is not to create a competing payroll system but to make discrepancies identifiable while memories and schedules are still available.
If money appears missing, contractual exclusion periods may matter. General information is not a substitute for prompt individual advice where a deadline may be running.