Re: [PROPOSAL] time zones and reproducible builds (was: How to make groff use local timezone?)

[Deri]

On Tuesday, 22 December 2020 00:24:15 GMT Dave Kemper wrote:
> Are there cases where the timestamp within the file would differ from
> the timestamp *of* the file?  This typically happens when a file is
> edited after it's created -- which could be common with HTML output,
> but would seem to be rare with PDF output.  Any tools that "edit" PDF
> files generally write out a new PDF file as a result, rather than
> editing the old one in place, right?

Not necessarily. The PDF 1.4 reference says:-

===================================================================================

3.4 File Structure
The preceding sections describe the syntax of individual objects. This section
describes how objects are organized in a PDF file for efficient random access 
and
incremental update. A canonical PDF file initially consists of four elements 
(see
Figure 3.2):

• A one-line header identifying the version of the PDF specification to which 
the
file conforms
• A body containing the objects that make up the document contained in the file
• A cross-reference table containing information about the indirect objects in 
the
file
• A trailer giving the location of the cross-reference table and of certain 
special
objects within the body of the file

This initial structure may be modified by later updates, which append additional
elements to the end of the file; see Section 3.4.5, “Incremental Updates,” for
details.

====================================================================================

So incremental updates of the file are catered for. In these files the creation 
date is retained and a ModDate is added to show date of latest amendment. Each 
object in the file has an object number and a generation number (initially 
zero) which allows for incremental amendment by adding to the generation number.

Cheers 

Deri