die Dienstzeit
working hours
The fixed period of time during which a person performs their professional duties or must be present for work; working hours.
Meine Dienstzeit endet heute um 17:00 Uhr.
My working hours end today at 5:00 p.m.

period of service / working hours
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working hours
The fixed period of time during which a person performs their professional duties or must be present for work; working hours.
Meine Dienstzeit endet heute um 17:00 Uhr.
My working hours end today at 5:00 p.m.

The word is made up of 'der Dienst' (service/duty) and 'die Zeit' (time). So it literally describes the time during which one performs one's duty or works.
Während seiner Dienstzeit ist er telefonisch nicht privat erreichbar.
During his working hours, he cannot be reached by phone for private matters.
'Dienstzeit' (period of service/working hours) is a rather formal and official term. It is often used in public service (for civil servants, police officers, soldiers) or in formal employment contracts.
Die genauen Dienstzeiten sind im Arbeitsvertrag geregelt.
The exact working hours are regulated in the employment contract.
You can recognize the meaning from the context. If it is about the daily schedule with times, it means daily working hours. If it is about an entire career, often with numbers of years, it means the total period of service.
Seine Dienstzeit ist von 9 bis 17 Uhr.
His working hours are from 9 a.m. to 5 p.m.
In connection with daily working hours, fixed phrases such as 'die Dienstzeit beginnt/endet' (working hours begin/end), 'während der Dienstzeit' (during working hours), or 'außerhalb der Dienstzeit' (outside working hours) are common.
Private Anrufe sind während der Dienstzeit nicht gestattet.
Private calls are not permitted during working hours.
When the total length of a career is meant, the word is often combined with verbs such as 'ableisten' (to complete, especially in the military), 'absolvieren' (to complete), or 'beenden' (to end). The phrase 'nach einer Dienstzeit von ... Jahren' (after a period of service of ... years) is also very typical.
Nach einer Dienstzeit von 25 Jahren erhielt sie eine besondere Auszeichnung.
After a period of service of 25 years, she received a special award.
free time
'die Freizeit' and 'die Dienstzeit' are clear opposites. 'Freizeit' is the time when you do not have to work and can decide for yourself what to do. 'Dienstzeit' is the fixed time during which someone works or must be present for work. So the difference is: in your free time, you are free to make your own plans; during working hours, you carry out professional duties. 'Dienstzeit' sounds more formal than 'Arbeitszeit' and is especially suitable for jobs with fixed shifts or duty periods.

Während der Dienstzeit darf Herr Kaya sein privates Handy nur in der Pause benutzen.
During working hours, Mr. Kaya may use his personal phone only during his break.
In ihrer Freizeit geht Anna gern schwimmen oder trifft Freunde.
In her free time, Anna likes to go swimming or meet friends.