I'll be happy if you help me out with the following question.
What does it imply - when we talk about an action that happened in the past using the past simple tense with a time expression like in the sentence "I washed the windows at 6 o'clock yesterday",
A) I started to wash windows at 6
B) At 6 I was in the middle of washing
C) I finished washing at 6
A careful native speaker would only use the simple past, with a specific time, about an action that was either instant or complete at the time mentioned.
I left my house at six o'clock
I arrived at work at nine o'clock
I went to bed at midnight.
John stopped the car at three fifteen in the afternoon.
The king died at five forty-three exactly.
Since washing (or "cleaning" as we say in Britain) windows cannot be done instantly, if a native speaker said "I washed the windows at six o'clock" to someone, then that listener would probably want to ask the same questions that you have.