Add getBlockTime rpc api (#7130)

* Add getBlockTime rpc api

* Add getBlockTime to docs

* Fix duration rounding for common tick/slot durations; add slot duration calculation

* Expose slots_per_year

* Use genesis values instead of clock defaults to calculate block offset

* Add get-block-time cli subcommand

* Fix test_rent: decrease magic number usage

sdk/src/timing.rs

@@ -38,27 +38,58 @@ pub fn years_as_slots(years: f64, tick_duration: &Duration, ticks_per_slot: u64)
         / ticks_per_slot as f64
 }
 
+/// From slots per year to tick_duration
+pub fn slot_duration_from_slots_per_year(slots_per_year: f64) -> Duration {
+    // Regarding division by zero potential below: for some reason, if Rust stores an `inf` f64 and
+    // then converts it to a u64 on use, it always returns 0, as opposed to std::u64::MAX or any
+    // other huge value
+    let slot_in_ns = (SECONDS_PER_YEAR * 1_000_000_000.0) / slots_per_year;
+    Duration::from_nanos(slot_in_ns as u64)
+}
+
 #[cfg(test)]
 mod test {
     use super::*;
 
     #[test]
     fn test_years_as_slots() {
-        let tick_duration = Duration::from_millis(1000 / 160);
+        let tick_duration = Duration::from_micros(1000 * 1000 / 160);
 
         // interestingly large numbers with 160 ticks/second
         assert_eq!(years_as_slots(0.0, &tick_duration, 4) as u64, 0);
         assert_eq!(
             years_as_slots(1.0 / 12f64, &tick_duration, 4) as u64,
-            109_572_659
+            105_189_753
         );
-        assert_eq!(years_as_slots(1.0, &tick_duration, 4) as u64, 1_314_871_916);
+        assert_eq!(years_as_slots(1.0, &tick_duration, 4) as u64, 1_262_277_039);
 
-        let tick_duration = Duration::from_millis(1000);
+        let tick_duration = Duration::from_micros(1000 * 1000);
         // one second in years with one tick per second + one tick per slot
         assert_eq!(
             years_as_slots(1.0 / SECONDS_PER_YEAR, &tick_duration, 1),
             1.0
         );
     }
+
+    #[test]
+    fn test_slot_duration_from_slots_per_year() {
+        let slots_per_year = 1_262_277_039.0;
+        let ticks_per_slot = 4;
+
+        assert_eq!(
+            slot_duration_from_slots_per_year(slots_per_year),
+            Duration::from_micros(1000 * 1000 / 160) * ticks_per_slot
+        );
+        assert_eq!(
+            slot_duration_from_slots_per_year(0.0),
+            Duration::from_micros(0) * ticks_per_slot
+        );
+
+        let slots_per_year = SECONDS_PER_YEAR;
+        let ticks_per_slot = 1;
+        assert_eq!(
+            slot_duration_from_slots_per_year(slots_per_year),
+            Duration::from_millis(1000) * ticks_per_slot
+        );
+    }
 }