daterange migration.yml

-  field_date: field_date
+  field_date:
+    plugin: iterator
+    source: field_date
+    process:
+      value: value
+      end_value: value2

Here's an example tested on Drupal 8.4.5 that imports start and end times, and adds 4 hours in the process.

langcode: en
status: true
id: events_csv_import
label: 'Import events'
source:
  plugin: csv
  path: sites/default/files/events.csv
  delimiter: ','
  enclosure: '"'
  header_row_count: 1
  keys:
    - id
  column_names:
    - id: 'Unique Id'
    - title: Title
    - body: 'Post body'
    - start: start
    - end: end
process:
  title: title
  body: body
  field_time_range/value:
    plugin: format_date
    source: start
    from_format: 'Y-m-d\TH:i:s'
    to_format: 'Y-m-d\TH:i:s'
    from_timezone: PST
    to_timezone: PST
  field_time_range/end_value:
    plugin: format_date
    source: end
    from_format: 'Y-m-d\TH:i:s'
    to_format: 'Y-m-d\TH:i:s'
    from_timezone: PST
    to_timezone: PST
  type:
    plugin: default_value
    default_value: event
destination:
  plugin: 'entity:node'
migration_dependencies: null

And here's a sample row from my CSV:

id	title	body	start	end
1	Meeting on Jan 8	This meeting is really important!	2018-01-08T09:00:00	2018-01-08T11:00:00

You might have to use the data_format process plugin to reformat the start and end dates to the proper format for the destination.

That works! Updated my code above.

One last issue I'm having: No matter what I try with from_timezone and to_timezone, and with or without -0800 (Pacfiic Standard Time) appended on the end of the CSV values, it always takes seems to add 4 hours.

@heddn please let me know if you have any timezone tips. Thanks for your help.