noun
reporting time; clock-in time
The time one is expected to arrive at the workplace and start work.
出勤時間は9時です。
The reporting time is 9 o'clock.
noun
time one leaves home for work
The time one departs from home to commute to work, rather than the official start time.
朝の出勤時間はだいたい7時半です。
My time for leaving home for work in the morning is around 7:30.
退社時間 is the time one leaves the office, while 出勤時間 is the time one arrives at work or leaves home for work.
Compound of 出勤 (going to work) and 時間 (time).