Mention numeric offsets from epoch

datetimes.Rmd

@@ -160,6 +160,13 @@ You may want to switch between a date-time and a date. That's the job of `as_dat
 as_date(now())
 ```
 
+Sometimes you'll get date/times as numeric offsets from the "Unix Epoch", 1970-01-01. If the offset is in seconds, use `as_datetime()`; if it's in days, use `as_date()`.
+
+```{r}
+# as_datetime(60 * 60 * 10)
+as_date(365)
+```
+
 ### Exercises
 
 1.  What happens if you parse a string that contains invalid dates?