This was a comment about writing English that I expanded for use here. The comment was for a document I was editing.

Time units and introductory words should be followed by commas. Examples:

Therefore,

However,

In addition,

Thursday afternoon,

Next week,

On Monday,

In the dark, the boy looked out from his campsite in the woods.

At night,

Alone in the woods,

Sometimes,

Anyway, I waited for the train, alone, as the snow blanketed the landscape, at midnight. As the moments passed, the snow deepened. However, a warm south wind warmed the night, nearly silently. At 12:14, on time, the train whistle blew, announcing its arrival. Impatiently, I scanned the tracks, but saw no light. Tomorrow, I'd be in Shanghai.  Tonight, would I leave Beijing.  I didn't know, yet.