```
data(sample_matrix)
endpoints(sample_matrix)
endpoints(sample_matrix, "weeks")
### example of how periods are based on the UNIX epoch,
### *not* the first observation of the data series
x <- xts(1:38, yearmon(seq(2018 - 1/12, 2021, 1/12)))
# endpoints for the end of every other year
ep <- endpoints(x, "years", k = 2)
# Dec-2017 is the end of the *first* year in the data. But when you start from
# Jan-1970 and use every second year end as your endpoints, the endpoints are
# always December of every odd year.
x[ep, ]
```

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