[R] shift/lag when merge zoo

[j.k]

Hello alltogheter,
I have a little problem regarding merging to zoo series.
I want to merge two zoo series to reduce the timegaps between the stamps.
I use the following code:

data.test <-
as.POSIXct(seq(data.input01[1,1],data.input01[nrow(data.input01),1],900),tz="GMT")
data.troughput01 <- as.zoo(data.input01$V2)
index(data.troughput01) <- as.POSIXct(data.input01$V1,tz="GMT")
data.output01 <-merge(data.troughput01,zoo(,data.test))

They look like that:

head(data.test)
[1] "2008-01-01 00:00:00 GMT" "2008-01-01 00:15:00 GMT" "2008-01-01 00:30:00
GMT"
[4] "2008-01-01 00:45:00 GMT" "2008-01-01 01:00:00 GMT" "2008-01-01 01:15:00
GMT"

> head(data.troughput01)
2008-01-01 00:00:00 2008-01-01 00:30:00 2008-01-01 01:00:00 2008-01-01
01:30:00 
           12.24180            11.27340            10.30500            
9.33654 
2008-01-01 02:00:00 2008-01-01 02:30:00 
            8.36811             7.62456 

> head(data.output01)
2008-01-01 01:00:00 2008-01-01 01:15:00 2008-01-01 01:30:00 2008-01-01
01:45:00 
            12.2418                  NA             11.2734                 
NA 
2008-01-01 02:00:00 2008-01-01 02:15:00 
            10.3050                  NA 


Are there any ideas why I have a lag of one hour?

At last I fill the NAs with na.approx:

data.output01 <-merge(data.troughput01,zoo(,data.test))

> head(data.output01)
2008-01-01 01:00:00 2008-01-01 01:15:00 2008-01-01 01:30:00 2008-01-01
01:45:00 
           12.24180            11.75760            11.27340           
10.78920 
2008-01-01 02:00:00 2008-01-01 02:15:00 
           10.30500             9.82077 
> 

Maybe there are suggestions for other solutions of achieving the increase of
the resolution.

Thanks in advance
Johannes

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