Converting a date to a week number and vice versa

This tip shows you how to convert a date to a week number and vice versa using Formula Language. It was written at R4, and I can no longer remember why it works -- but it does.

The International Organization for Standardization defines Week 1 in a given year as the first week (starting Monday) that contains at least 4 January-days. In other words, Week 1 is the first week that includes a Thursday.

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Klavs Thor Olesen says in in his tip that LotusScript can return a Week Number for a Date directly.

I would like to know how, because I have not found any method that does this.
—Goran L.

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It is the Format function I referred to. Here is an example:
—Klavs Thor Olesen, tip author

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This is exactly what I was looking for. I was looking for a formula to calculate a week number field on a form based on a date entered in another field. I typed in calculate week number in a search engine and there it was, number eight in my search result...

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s. It's just what I was looking for and it does exactly what I need it to do. I set the properties of the new field to compute after validation and it works perfectly. Thanks very much.
—Bob V.

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Here is the same function converted to LotusScript:

Also, here's a handy hint for the Formula code, using correct weekdays:

I hope this helps (I've got a calendar population routine in the pipeline that I shall be submitting later). A huge thanks to Klavs for this function; it's a lifesaver!
—Andrew D.

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