[Logs UI] Log Analysis screen turns "now" date into fixed timestamp

[weltenwort]

Kibana version: 7.4 (at 69c9ba5)

Original install method: source

Describe the bug:

Selecting the "now" date as either start or end date in the Logs Analysis page's date picker and clicking "Update" turns the "now" into the current timestamp. Reloading the page or clicking the "Refresh" button causes the same time interval to be loaded repeatedly without "now" adjusting to the current time.

Steps to reproduce:

1. Navigate to the Logs Analysis page.
2. Select "now" as the end date.
3. Click "Update".
4. Observe how the "now" is replaced with a specific timestamp.
5. Observe how reloading the page or clicking "Refresh" does not advance the end date.

Expected behavior:

The interval boundary selected as "now" adjusts to match the current time on every refresh or page reload.

Any additional context:

The state used to persist the dates is a number. In addition to fixed timestamps relative date math expressions need to be supported.

[elasticmachine]

Pinging @elastic/infra-logs-ui