Reshaping data with xtabs giving me 'extra' data

Dear all,

Lets say I have several data frames as follows:

set.seed(42)
dates <- as.Date(c("2010-01-19", "2010-01-20"))
times <- c("09:30:00", "11:30:00", "13:30:00", "15:30:00")
shows <- c("Red Dwarf", "Being Human", "Doctor Who")

df1 <- data.frame(Date = dates[1], Time = times[1], Show = shows, Score = 1:3)
df2 <- data.frame(Date = dates[1], Time = times[2], Show = shows, Score = 1:3)
df3 <- data.frame(Date = dates[1], Time = times[4], Show = shows, Score = 1:3)
df4 <- data.frame(Date = dates[2], Time = times[1], Show = shows, Score = 1:3)
df5 <- data.frame(Date = dates[2], Time = times[2], Show = shows, Score = 1:3)
df6 <- data.frame(Date = dates[2], Time = times[3], Show = shows, Score = 1:3)
df7 <- data.frame(Date = dates[2], Time = times[4], Show = shows, Score = 1:3)
df7

? ? ? ?Date ? ? Time ? ? ? ?Show Score
1 2010-01-20 15:30:00 ? Red Dwarf ? ? 1
2 2010-01-20 15:30:00 Being Human ? ? 2
3 2010-01-20 15:30:00 ?Doctor Who ? ? 3

I would like to somehow reshape the data into a different format:

df.list <- list(df1, df2, df3, df4, df5, df6, df7)
my.df <- Reduce(function(x, y) merge(x, y, all=TRUE), df.list, accumulate=F)
my.xtab <- xtabs(as.numeric(Score) ~ Date + Show + Time, data = my.df)

This is where my problem occurs. In Time = 13:30:00, there is now data
for "2010-01-19" which was not in any of my original data frames
above:

# I do not want the zeros below
my.xtab[,,"13:30:00"]

? ? ? ? ? ?Show
Date ? ? ? ? Being Human Doctor Who Red Dwarf
?2010-01-19 ? ? ? ? ? 0 ? ? ? ? ?0 ? ? ? ? 0
?2010-01-20 ? ? ? ? ? 2 ? ? ? ? ?3 ? ? ? ? 1

Perhaps I am missing something in the way i call the xtabs function?

Thank you kindly for your time,
Tony Breyal

OS: Windows XP 64bit

sessionInfo()

R version 2.10.0 (2009-10-26)
i386-pc-mingw32

locale:
[1] LC_COLLATE=English_United States.1252 ?LC_CTYPE=English_United
States.1252 ? ?LC_MONETARY=English_United States.1252
LC_NUMERIC=C ? ? ? ? ? ? ? ? ? ? ? ? ? LC_TIME=English_United States.
1252

attached base packages:
[1] stats ? ? graphics ?grDevices utils ? ? datasets ?methods
base

loaded via a namespace (and not attached):
[1] tools_2.10.0