In ASP.NET, the clientCache > cacheControlMaxAge property is a timespan (though the HTTP specification makes its max-age header seconds.)
So, this is wrong:
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="86400"/>
And this is right:
<clientCache cacheControlMode="UseMaxAge" cacheControlMaxAge="1.00:00:00"/>
Where was the ISO on this one?