Timestamp off-chain API (#3144)

* timestamp offchain api impl

* Addressed review comments

* fixed expect message

* fixed compile error

* fixed line width

substrate/core/offchain/src/api.rs

@@ -14,7 +14,12 @@
 // You should have received a copy of the GNU General Public License
 // along with Substrate.  If not, see <http://www.gnu.org/licenses/>.
 
-use std::{str::FromStr, sync::Arc, convert::TryFrom};
+use std::{
+	str::FromStr, 
+	sync::Arc, 
+	convert::{TryFrom, TryInto}, 
+	time::SystemTime
+};
 use client::backend::OffchainStorage;
 use crate::AuthorityKeyProvider;
 use futures::{Stream, Future, sync::mpsc};
@@ -308,7 +313,20 @@ where
 	}
 
 	fn timestamp(&mut self) -> Timestamp {
-		unavailable_yet("timestamp")
+		let now = SystemTime::now();
+		let epoch_duration = now.duration_since(SystemTime::UNIX_EPOCH);
+		match epoch_duration {
+			Err(_) => {
+				// Current time is earlier than UNIX_EPOCH.
+				Timestamp::from_unix_millis(0)
+			},
+			Ok(d) => {
+				let duration = d.as_millis();
+				// Assuming overflow won't happen for a few hundred years.
+				Timestamp::from_unix_millis(duration.try_into()
+					.expect("epoch milliseconds won't overflow u64 for hundreds of years; qed"))
+			}
+		}
 	}
 
 	fn sleep_until(&mut self, _deadline: Timestamp) {
@@ -574,6 +592,22 @@ mod tests {
 		AsyncApi::new(pool, db, "pass".to_owned().into(), TestProvider::default(), BlockId::Number(Zero::zero()), mock)
 	}
 
+	#[test]
+	fn should_get_timestamp() {
+		let mut api = offchain_api().0;
+		
+		// Get timestamp from std.
+		let now = SystemTime::now();
+		let d: u64 = now.duration_since(SystemTime::UNIX_EPOCH).unwrap().as_millis().try_into().unwrap();
+
+		// Get timestamp from offchain api.
+		let timestamp = api.timestamp();
+		
+		// Compare.
+		assert!(timestamp.unix_millis() > 0);
+		assert_eq!(timestamp.unix_millis(), d);
+	}
+
 	#[test]
 	fn should_set_and_get_local_storage() {
 		// given