0) Summary

The below looks at 2 questions:

1. Updated Question (DATETIME_DIFF): Find the difference between the two time fields, NewStartTime and NewEndTime and also incorporate a Date field;
2. Original Question (TIME_DIFF): Looks for the difference between 2 Time fields, StatTime and EndTime.

1) Update (17 Sep 2020 Dates & Time Update)

Updated the Answer with the solution using the Updated Date Time Functions which incorporates the PARSE_DATETIME and DATETIME_DIFF functions:

1.1) Upgrade the Date Field

Upgrade the Date field to the newer Date field type and ensure that the NewStartTime and NewEndTime fields are set to Text.

Added a GIF to elaborate:

1.2) DATETIME_DIFF

Copy-paste the Calculated Field below to create a value in seconds that shows the difference between the two fields:

DATETIME_DIFF(PARSE_DATETIME("%Y/%m/%d%I:%M:%S %p",CONCAT(Date,NewEndTime)), PARSE_DATETIME("%Y/%m/%d%I:%M:%S %p",CONCAT(Date,NewStartTime)), SECOND)

1.3) Type (DATETIME_DIFF)

Number > Duration (Sec.)

Added a New Page to the Report and a GIF to demonstrate:

2) Original Post

It can be achieved using the 3 steps below:

2.1) Type (HH:MM Fields)

By default, the fields should be detected as Text fields, if not ensure that they are set to Text fields at the Data Source, such that it looks like:

2.2) Time_DIFF

Copy-paste the Calculated Field below to create a value in seconds that shows the difference between the two fields:

((CAST(REGEXP_EXTRACT(EndTime,"^(\\d+):")AS NUMBER)*60*60) + (CAST(REGEXP_EXTRACT(EndTime,"^\\d+:(\\d+)")AS NUMBER)*60) + NARY_MAX(CAST(REGEXP_REPLACE(EndTime,".*(PM)$","43200")AS NUMBER),0)) -
((CAST(REGEXP_EXTRACT(StatTime,"^(\\d+):")AS NUMBER)*60*60) + (CAST(REGEXP_EXTRACT(StatTime,"^\\d+:(\\d+)")AS NUMBER)*60) + NARY_MAX(CAST(REGEXP_REPLACE(StatTime,".*(PM)$","43200")AS NUMBER),0))

2.3) Type (Time_DIFF)

Numeric > Duration (Sec.)

Google Data Studio Report and a GIF to elaborate: