title: Is it possible to make a date with negative year?
body: <p>I have this data I wanted to convert to dates, but I doubt it is possible with year that is below 0, below is the snippets</p>
  
  <pre><code>library(datasets)
  library(quantmod)
  data(treering)
  tree_df = data.frame(ds=index(treering), y=as.numeric(treering))
  &gt; head(tree_df)
       ds     y
  1 -6000 1.345
  2 -5999 1.077
  3 -5998 1.545
  4 -5997 1.319
  5 -5996 1.413
  6 -5995 1.069
  &gt; tail(tree_df)
         ds     y
  7975 1974 1.031
  7976 1975 1.027
  7977 1976 1.173
  7978 1977 1.471
  7979 1978 1.444
  7980 1979 1.160
  
  ?treering
      Yearly Treering Data, -6000–1979
      Description
      Contains normalized tree-ring widths in dimensionless units.
  
      Usage
      treering
      Format
      A univariate time series with 7981 observations. The object is of class "ts".
  
      Each tree ring corresponds to one year.
  </code></pre>
  
  <p>Is there a way to convert the data into dates with a negative year in its own way? like for example "-6000-01-01"?</p>