[amibroker] One extra bar when using 5min bars from 1min data?

[ezbentley]

My data base has 1 minute resolution, but when I set the interval to 5 minutes, 
the first bar of the day appears as 9:31. The second bar appears as 9:35 and 
the rest of the day is normal. Due to that additional bar at open, there are 79 
5-minute bars in a day instead of 78. Why is that? I tried to play with the 
Preferences->Intraday settings, but those settings just change the time stamp 
for each bar and don't fix the problem that I got two bars at open instead of 
one. 
Can anyone help?