# Thread: The fair value of the index at 31 December

1. ## The fair value of the index at 31 December

The sentence below is from one of my textbook. Why shoudn't we write "on 31 December" instead of "at 31 December"?

Sorenson calculates the fair value of the index at 31 December.

2. ## Re: The fair value of the index at 31 December

The writer of the sentence had good reason to use at instead of on. His choice of at suggests that Sorenson was tracking the value of the index across a period of time, possibly several weeks or months. 31st December is a point along this track.

We tend to use on with days to talk about events which happen within the duration of that day.

3. ## Re: The fair value of the index at 31 December

Thanks, I think I get it.

It's sort of unrelated but since you use "across a period of time", I ask myself if there are any differences in meaning between "across a period of time" and "over a period of time"?

4. ## Re: The fair value of the index at 31 December

Yes, there can be differences in use/meaning. In my sentence above I could have used over to say essentially the same thing, but there was a reason why I chose to use across instead of over.

5. ## Re: The fair value of the index at 31 December

Well, what was that reason?

Does "across" put more emphasis on the boundaries of the period than "over"?

6. ## Re: The fair value of the index at 31 December

Since I used the verb "tracking", I considered across as better expressing a sense of movement through (temporal) space from a beginning point to an end point.

Let me offer a way to begin thinking about the very subtle differences of emphasis between across and over as prepositions of space/time:

• They travelled across the sea. (emphasises the crossing or the getting there)
• They travelled over the sea. (emphasises the completion or the arriving there)

Another way that I understand it is that over has a sense of the other side whereas across has a sense of the distance between.

In my mind's eye, Sorenson began calculating at one point (say, April), continued past December and finished at an end point (April) the following year. He was on a journey.

#### Posting Permissions

• You may not post new threads
• You may not post replies
• You may not post attachments
• You may not edit your posts
•