Simplify nextRecurDate interface

Make nextRecurDate always return the same object type, rather than
sometimes returning a Date and sometimes returning its own custom
object format, to simplify the code that uses it.

background.js

@@ -553,14 +553,9 @@ const SendLater = {
     }
 
     if (nextRecur) {
-      let nextRecurAt, nextRecurSpec, nextRecurArgs;
-      if (nextRecur.getTime) {
-        nextRecurAt = nextRecur;
-      } else {
-        nextRecurAt = nextRecur.sendAt;
-        nextRecurSpec = nextRecur.nextspec;
-        nextRecurArgs = nextRecur.nextargs;
-      }
+      let nextRecurAt = nextRecur.sendAt;
+      let nextRecurSpec = nextRecur.nextspec;
+      let nextRecurArgs = nextRecur.nextargs;
       while (nextRecurAt < new Date()) {
         nextRecurAt = new Date(nextRecurAt.getTime() + 60000);
       }

test/nextrecurtests.js

@@ -11,10 +11,10 @@ exports.init = function () {
       return "Unexpected error: " + ex;
     }
     expected = new Date(expected);
-    if (result.getTime() == expected.getTime()) {
+    if (result.sendAt.getTime() == expected.getTime()) {
       return true;
     } else {
-      return `Expected ${expected}, got ${result}`;
+      return `Expected ${expected}, got ${result.sendAt}`;
     }
   }
 
@@ -26,7 +26,7 @@ exports.init = function () {
         recur,
         new Date(now),
       );
-      return "Expected exception, got " + result;
+      return "Expected exception, got " + result.sendAt;
     } catch (ex) {
       if ((ex + "").indexOf(expected) !== -1) {
         return true;

utils/static.js

@@ -1498,7 +1498,7 @@ var SLStatic = {
       return null;
     }
 
-    return next;
+    return { sendAt: next };
   },
 
   // sendAt is a Date object for the scheduled send time we need to adjust.