[R] Reading in irregular, daily time series data

[veol]

Hello,

I am trying to read in some time series data and am having trouble. Forgive
me, I am quite new to R. The data is in the form of:

"TS.1"
"2000-07-28" 1419.89
"2000-07-31" 1430.83
"2000-08-01" 1438.1
"2000-08-02" 1438.7
"2000-08-03" 1452.56
"2000-08-04" 1462.93
"2000-08-07" 1479.32
"2000-08-08" 1482.8
"2000-08-09" 1472.87
"2000-08-10" 1460.25
"2000-08-11" 1471.84
"2000-08-14" 1491.56
"2000-08-15" 1484.43
...

The data is daily data, but it is irregular (i.e. there are some missing
data points). 

I have tried reading in the data like so, but when I plot, the time series
does not preserve the dates.

sp500 <- ts(read.table("SP500.txt",header=TRUE))
plot.ts(sp500)

I have searched the forums, and have found the current method to work:

sp500 <- read.table("SP500.txt",header=TRUE)
date <- sp500[,1]
data <- sp500[,2]
z <- aggregate(zoo(data), as.Date(date), tail, 1)
merge(z, zoo(, as.Date(unclass(time(as.ts(z))), fill=0)))
plot.zoo(z)

However, now I cannot analyze the acf of the time series, as R complains
giving an error:
Error in na.fail.default(as.ts(x)) : missing values in object

Is there any way to fix this?

I would actually prefer not using the zoo class, as it breaks the acf
function in R. Would the new zoo object be compatible with all the ts
functions?

Thank you!

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