How to show the age of the oldest Req in a Dashboard

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Hi,

I'm beginning with OTBI and Taleo, so sorry for the noob questions

I wanted to create a dashboard showing the number of opened Req (which I could do with a performance Tile link to the Req count and a filter on the req status on sourcing), and next to it the number of days since the oldest opened req was opened.

I'm at a loss for doing that, I suspect the selection step could help me, but ...

any help or example I could follow would be welcome,

thanks in advance,

Damien