fix(timer): multiple subscriptions to timer(Date) behaves correctly

- Moves the calculation of the due time to the subscription phase of the Observable
- Adds a test confirming multiple subscriptions on delay

fixes #3252

spec/observables/timer-spec.ts

@@ -1,10 +1,12 @@
 import marbleTestingSignature = require('../helpers/marble-testing'); // tslint:disable-line:no-require-imports
-import { timer, never } from '../../src/create';
+import { timer, never, merge } from '../../src/create';
 import { TestScheduler } from '../../src/testing';
+import { mergeMap } from '../../src/operators';
 
 declare const asDiagram: any;
 declare const time: typeof marbleTestingSignature.time;
 declare const expectObservable: typeof marbleTestingSignature.expectObservable;
+declare const cold: typeof marbleTestingSignature.cold;
 declare const rxTestScheduler: TestScheduler;
 
 /** @test {timer} */
@@ -89,4 +91,21 @@ describe('timer', () => {
     const values = { a: 0, b: 1, c: 2, d: 3, e: 4};
     expectObservable(source).toBe(expected, values);
   });
+
+  it('should still target the same date if a date is provided even for the ' +
+    'second subscription', () => {
+      const offset = time('----|    ');
+      const t1 = cold(    'a|       ');
+      const t2 = cold(    '--a|     ');
+      const expected =    '----(aa|)';
+
+      const dueTime = new Date(rxTestScheduler.now() + offset);
+      const source = timer(dueTime, null, rxTestScheduler);
+
+      const testSource = merge(t1, t2).pipe(
+        mergeMap(() => source)
+      );
+
+      expectObservable(testSource).toBe(expected, {a: 0});
+  });
 });

src/internal/observable/timer.ts

@@ -62,13 +62,15 @@ export function timer(dueTime: number | Date = 0,
     scheduler = async;
   }
 
-  const due = isNumeric(dueTime)
-    ? (dueTime as number)
-    : (+dueTime - scheduler.now());
+  return new Observable(subscriber => {
+    const due = isNumeric(dueTime)
+      ? (dueTime as number)
+      : (+dueTime - scheduler.now());
 
-  return new Observable(subscriber => scheduler.schedule(dispatch, due, {
-    index: 0, period, subscriber
-  }));
+    return scheduler.schedule(dispatch, due, {
+      index: 0, period, subscriber
+    });
+  });
 }
 
 interface TimerState {