wizard: allow calendar date for restoration height

js/Wizard.js

@@ -177,3 +177,46 @@ function restoreWalletCheckViewSpendAddress(walletmanager, nettype, viewkey, spe
 
     return addressOK && viewKeyOK && spendKeyOK
 }
+
+//usage: getApproximateBlockchainHeight("March 18 2016") or getApproximateBlockchainHeight("2016-11-11")
+//returns estimated block height with 1 month buffer prior to requested date.
+function getApproximateBlockchainHeight(_date){
+    // time of monero birth 2014-04-18 10:49:53 (1397818193)
+    var moneroBirthTime = 1397818193;
+    // avg seconds per block in v1
+    var secondsPerBlockV1 = 60;
+    // time of v2 fork 2016-03-23 15:57:38 (1458748658)
+    var forkTime = 1458748658;
+    // v2 fork block
+    var forkBlock = 1009827;
+    // avg seconds per block in V2
+    var secondsPerBlockV2 = 120;
+    // time in UTC
+    var requestedTime = Math.floor(new Date(_date) / 1000);
+    var approxBlockchainHeight;
+    var secondsPerBlock;
+    // before monero's birth
+    if (requestedTime < moneroBirthTime){
+        console.log("Calculated blockchain height: 0, requestedTime < moneroBirthTime " );
+        return 0;
+    }
+    // time between during v1
+    if (requestedTime > moneroBirthTime && requestedTime < forkTime){
+        approxBlockchainHeight = Math.floor((requestedTime - moneroBirthTime)/secondsPerBlockV1);
+        console.log("Calculated blockchain height: " + approxBlockchainHeight );
+        secondsPerBlock = secondsPerBlockV1;
+    }
+    // time is during V2
+    else{
+        approxBlockchainHeight =  Math.floor(forkBlock + (requestedTime - forkTime)/secondsPerBlockV2);
+        console.log("Calculated blockchain height: " + approxBlockchainHeight );
+        secondsPerBlock = secondsPerBlockV2;
+    }
+    var blocksPerMonth = 60*60*24*30/secondsPerBlock;
+    if(approxBlockchainHeight - blocksPerMonth > 0){
+        return approxBlockchainHeight - blocksPerMonth;
+    }
+    else{
+        return 0;
+    }
+}