Bug 1089564 - Always keep latest time range in week view when come back from other screens

I would rewrite as:

```js
// if returning from another screen we should not change the date
if (this.baseDate && Calc.isSameDate(date, this.baseDate)) {
  return this.baseDate;
}
// show monday as the first day of the grid if date is between Mon-Fri
var index = Calc.dayOfWeekFromMonday(date.getDay());
if (index < 5) {
  date = Calc.createDay(date, date.getDate() - index);
}
return date;
```

that way we avoid calculating the `index` if not needed and decouples both behaviors (easier to understand as 2 separate conditionals).