Remove deprecated chrono functions. (#1199)

This PR replaces a now deprecated use of chrono with its current equivalent.

src/commons/api/history.rs

@@ -1,6 +1,6 @@
 use std::{collections::BTreeMap, fmt};
 
-use chrono::{DateTime, NaiveDateTime, SecondsFormat, Utc};
+use chrono::{DateTime, SecondsFormat};
 
 use rpki::{
     ca::{
@@ -123,8 +123,9 @@ impl<E: Event, I: InitEvent> From<StoredEffect<E, I>> for CommandHistoryResult {
 impl CommandHistoryRecord {
     pub fn time(&self) -> Time {
         let seconds = self.timestamp / 1000;
-        let time = NaiveDateTime::from_timestamp_opt(seconds, 0).expect("timestamp out-of-range");
-        Time::from(DateTime::from_utc(time, Utc))
+        DateTime::from_timestamp(
+            seconds, 0
+        ).expect("timestamp out-of-range").into()
     }
 }