Function and CASE

I've been working round this for a while but decided to finally tackle it head on as it's becoming a regular pain.

I have a lot of customs functions that call upon others to get standing data. So for instance one formula works out the tax due on income. But rather than have the variables there they are in a separate function to make life easier to update it. One of these is for dates.

This is what it looks like now.

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In cell it is =update_dates("tax_year")

And this works. But I can't help thinking I'm missing something. Like I shouldn't need to use the case statement - I should be able to go directly from the variable "required_date" to the answer, even more so in this example as the defined constants and the names passed are the same. But I've never been able to get it to work. I would guess something like update_dates=required_date - but clearly that just gives the text that was passed to it.

Anyone any ideas? I've a feeling it's quite straightforward but I've missed it. Thanks in advance.

NUts! My reply vanished. SO the shorter version. Basically the constants might be used in 40 spreadsheets and I can't use a link to any external workbooks. SO the best way to sort it (and update it considering the sheets are protected) was thought to be via the VBA code.

As another for instance there is a formula =DC("what_rate_you_want","what_year"). As each year has about 60 rates it uses arrays instead, matching the text against the rate list to give an index. That index is then used in another array to give the magic number.

But this date one is simpler (thankfully). Just can't help thinking I'm missing something with it.