It’s not a number of years that gets celebrated in the English-speaking world. A touch of rephrasing or explanation is needed if you don’t want your readers scratching their heads.
It’s a silver wedding for 25 years, a golden wedding for 50 years and traditionally – though not really celebrated – there are also other materials such as paper, crystal, wood, copper and pearl for other numbers. But there’s no twelve and a half on the list.
- 12,5 jaar in dienst (long service award)
- 12,5 jaar getrouwd (wedding anniversary)
- 12,5 jaar jubileum (since the company was founded)
The numbers and materials for the smaller numbers – paper and cotton and so forth – can vary somewhat between the US and the UK.
Prevalence: low. In fact, this one didn’t make the cut into the top two hundred issues and wouldn’t have been the subject of an article until it coincidentally went and turned up twice yesterday.
Frequency: low. Yet to appear in more than one context in the same document.
Native: no. I remember being quite confused by it myself the first time it ever came up.
Advanced Dunglish 
And as it happens, another reason for this post is that Clare has now been working with me at Tessera Translations BV for exactly twelve and a half years today.
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Het twaalf en een half jarig jubileum is meestal niet since the company was founded maar since the employee entered service with them.
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*into service, probably?
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Agreed, it’s usually the “long service award” first mentioned, for a specific employee. I have seen the other, though!
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