SOURCE_DATE_EPOCH: Add R example

_docs/source-date-epoch.md

@@ -186,13 +186,13 @@ docker buildx build --output type=image,name=docker.io/username/image,push=true,
 
 The `rewrite-timestamp` option is not set to `true` by default due to the overhead of rewriting image layers.
 
-#### apt-get
+##### apt-get
 `RUN apt-get` does not automatically consume `SOURCE_DATE_EPOCH` to install packages from the past snapshot.
 
 <https://github.com/reproducible-containers/repro-sources-list.sh> can be used for reconfiguring `/etc/apt/sources.list`
 to use `https://snapshot.debian.org/archive/debian/<SOURCE_DATE_EPOCH>/`.
 
-#### Further information
+##### Further information
 See:
 - https://github.com/moby/buildkit/blob/master/docs/build-repro.md
 - https://github.com/docker-library/official-images/issues/16044
@@ -463,6 +463,16 @@ if (sourceDateEpoch != null) {
 def extendedTimestamp = buildInstant.toString()
 ```
 
+### R
+
+```r
+if (Sys.getenv("SOURCE_DATE_EPOCH") == "") {
+  current_date <- Sys.Date()
+} else {
+  current_date <- as.Date(as.numeric(Sys.getenv("SOURCE_DATE_EPOCH")) / 86400, "1970-01-01")
+}
+```
+
 
 ### Last-resort using faketime