How do I keep the date-format from Excel when importing a time series in R?

Question

When I import a financial time series from Excel to R, the dates turn into integers. Each column represents a date. I understand that Excel converted them to a form where they represent the number of days since the origin date of 01/01/1900. How can I turn these numbers back into 'normal' dates in R?

Answer 1

I would use one of these functions:

xlsnum_to_POSIXct <- function(n)
{
    as.POSIXct(
       x = n*24*60*60,
       origin = "1900-01-01 00:00:00"
    );
}

xlsnum_to_POSIXlt <- function(n)
{
    as.POSIXlt(
       x = n*24*60*60,
       origin = "1900-01-01 00:00:00"
    );
}

which converts the number of days into number of seconds since 1900/01/01, and wraps it conveniently into an R date-time type.

Should work also with fractions of day, vectors of day counts etc.