Creating a date or time index

So far, our focus in this chapter was mainly on the attributes of the date and time classes. Let's now connect the dots and see some useful applications of time series data. As introduced in Chapter 1, Introduction to Time Series Analysis and R, the main characteristic of time series data is its time index (or timestamp), an equally spaced time interval. The base package provides two pairs of functions, seq.Date and seq.POSIXt, to create a time index vector with Date or POSIX objects respectively. The main difference between the two functions (besides the class of the output) is the units of the time interval. It will make sense to use the seq.Date function to generate a time sequence with daily frequency or lower (for example, weekly, monthly, and so on) and as.POSIXt in other instances (for higher frequencies than daily, such as hourly, half...