chain: add getProofTime.

lib/blockchain/chain.js

@@ -1825,6 +1825,34 @@ Chain.prototype.getOrphanRoot = function getOrphanRoot(hash) {
   return root;
 };
 
+/**
+ * Calculate the time difference (in seconds)
+ * between two blocks by examining chainworks.
+ * @param {ChainEntry} to
+ * @param {ChainEntry} from
+ * @returns {Number}
+ */
+
+Chain.prototype.getProofTime = function getProofTime(to, from) {
+  var sign, work;
+
+  if (to.chainwork.cmp(from.chainwork) > 0) {
+    work = to.chainwork.sub(from.chainwork);
+    sign = 1;
+  } else {
+    work = from.chainwork.sub(to.chainwork);
+    sign = -1;
+  }
+
+  work = work.imuln(this.network.pow.targetSpacing);
+  work = work.idiv(this.tip.getProof());
+
+  if (work.bitLength() > 53)
+    return sign * util.MAX_SAFE_INTEGER;
+
+  return sign * work.toNumber();
+};
+
 /**
  * Calculate the next target based on the chain tip.
  * @returns {Promise} - returns Number