message: Monitoring: Fix silence form bug with handling invalid start times
  The end time was being calculated based on the start time even if the
  start time was invalid.
diff: @@ -1328,9 +1328,12 @@ const SilenceForm_: React.FC<SilenceFormProps> = ({ defaults, Info, title }) =>
  );
  const [startsAt, setStartsAt] = React.useState(defaults.startsAt ?? formatDate(now));
  - const durationEnd = formatDate(
  - new Date((isStartNow ? Date.now() : Date.parse(startsAt)) + parsePrometheusDuration(duration)),
  - );
  + const getEndsAtValue = (): string => {
  + const startsAtDate = Date.parse(startsAt);
  + return startsAtDate
  + ? formatDate(new Date(startsAtDate + parsePrometheusDuration(duration)))
  + : '-';
  + };
  const setMatcherField = (i: number, field: string, v: any): void => {
  const newMatchers = _.clone(matchers);
  @@ -1440,7 +1443,7 @@ const SilenceForm_: React.FC<SilenceFormProps> = ({ defaults, Info, title }) =>
  ) : (
  <DatetimeTextInput
  isDisabled
  - value={isStartNow ? `${duration} from now` : durationEnd}
  + value={isStartNow ? `${duration} from now` : getEndsAtValue()}
  />
  )}
  </div>
  