search: fix server-side date behaviour

OffsetMinutesFromUTC was being passed incorrectly by the web app
(getTimezoneOffset if UTC - Timezone, we wanted Timezone - UTC), now
fixed. This value is now used if given in comparisons. Times are
truncated to minute-deep precision, and Any date comparison ignores
empty date fields (i.e. a unix timestamp being 0 or a time.Time.IsZero()
== true).

ts/typings/d.ts

@@ -174,6 +174,7 @@ interface ParsedDate {
     attempt: DateAttempt;
     date: Date;
     text: string;
+    invalid?: boolean;
 };
 
 declare var config: Object;