Correlation between two asynchronous time series?

Message: 4
Date: Thu, 13 Nov 2008 21:53:14 -0800
From: Michael <comtech.usa at gmail.com>
Subject: [R-SIG-Finance] correlation between two asynchronous time
	series?
To: r-sig-finance at stat.math.ethz.ch
Message-ID:
	<b1f16d9d0811132153qd277378ka3d29b9d30a5d753 at mail.gmail.com>
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Hi all,

If I want to find out the correlation between two time series, one is
the Hong Kong stock index, the other is the S&P 500. The two markets
open at two different time.

What impact it might have on my estimate of the correlation of the two
series? 

How do I address this asynchronous time series problem? Any
good models?
  

Thanks a lot!