Rollup merge of rust-lang#44315 - kallisti5:epoch-doc-example-squashed, r=GuillaumeGomez

std/time: Give an example to get UNIX_EPOCH in seconds

Author: Mark-Simulacrum

src/libstd/time/mod.rs

@@ -382,6 +382,17 @@ impl fmt::Debug for SystemTime {
 /// [`SystemTime`] instance to represent another fixed point in time.
 ///
 /// [`SystemTime`]: ../../std/time/struct.SystemTime.html
+///
+/// # Examples
+///
+/// ```no_run
+/// use std::time::{SystemTime, UNIX_EPOCH};
+///
+/// match SystemTime::now().duration_since(UNIX_EPOCH) {
+///     Ok(n) => println!("1970-01-01 00:00:00 UTC was {} seconds ago!", n.as_secs()),
+///     Err(_) => panic!("SystemTime before UNIX EPOCH!"),
+/// }
+/// ```
 #[stable(feature = "time2", since = "1.8.0")]
 pub const UNIX_EPOCH: SystemTime = SystemTime(time::UNIX_EPOCH);