The timezone defenition used on the machine may not exist on machine where the report is generated, which can cause errors

[jobinau]

These are very edge cases. But happened once or twice.
For example, the User environment has the Timezone specified as "GB" (which stands for Great Briton), which exists in that environment.
However, when the analysis report was prepared on another region of the world , PostgreSQL couldn't know what "GB" meant, and it resulted in errors.
Another problem is pg_gather allows users to specify custom timezone specifications so that all timestamps will be displayed in the timezone they specify

export PG_GATHER_TIMEZONE='XYZ`

But if the user specifies a timezone which doesn't exist, PostgreSQL will error, and the report generation will fail.

In order to avoid both these cases, the pg_gather analysis and report generation should validate the timezone specified. if it is not a valid timezone, handle the situation without causing any failure.

[jobinau]

One straightforward solution possible is to detect the problem at the beginning and default the timezone to UTC if none of the specified time zones can be validated.

[jobinau]

Now, an Unidentifiable timezone specification, whether it is from the parameter or environment variable, Falls back to UTC.
This helps to prevent run-time errors and missing headers in the report.
Commit : 0b07dfe