Is 一日千秋 a chengyu? I have tried searching for it using google with mixed results.
If it is a chengyu, what is its grammatical function?
一日千秋 is a Japanese idiom. When I searched for the term on Google, almost all of the websites were in Japanese, and the top Chinese website explains it is a Japanese idiom.
According to the website above, 一日千秋 means:
This reads as:
The feeling that it takes too long for a day to end. The strong sentiment to wait for someting.
I think the 一日三秋 on the website is a typo...
Never heard of 一日千秋.
Most likely, you're thinking of the idiom "一日三秋" , which came from an expression "一日不見，如隔三秋" -- "Not seeing (each other) for just one day, feels like it has been three autumns (three years)"
"一日三秋" express the sentiment of "time seems moving painfully slow when you are waiting anxiously"