DATEADD function have severe performance degradation on linked server query
<p>I am running a query from our on premise SQL server which is linked to an azure database via a linked server and I run the below query in milliseconds:</p>

<pre><code>SELECT *
FROM [Cloud].[website].[dbo].[Topup]
WHERE CAST(senttovpn AS DATE) = '7 aug 2016'
GO
</code></pre>

<p>Due to the server time being 1 hour behind our current time I want to add an hour to the timestamp and so run the following query:</p>

<pre><code>SELECT *
FROM [Cloud].[website].[dbo].[Topup]
WHERE CAST(DATEADD(hour, 1, senttovpn) AS DATE) = '7 aug 2016'
GO
</code></pre>

<p>This however takes 1 minute and 19 seconds to run. Can anyone tell me why this degradation is so severe a what is the best SQL solution to this?</p>