Now, based on new information, we have revised that conclusion: The timezone offset was in fact GMT-4 (US Eastern DST). Here, we will describe how we arrived at this new, surprising conclusion and relate it to our prior work.
We mistakenly interpreted the last modified time that LibreOffice wrote as “2015-08-25T23:07:00Z” as a GMT time value. Typically, the trailing “Z” means “Zulu Time“, but in this case, LibreOffice incorrectly added the “Z”. McIntyre’s tests confirm that LibreOffice records the “last modified” time as local time (not GMT). The following section describes the method that we used to determine the timezone offset in force when the document was saved.
LibreOffice Leaks the Time Zone Offset in Force when a Document was Last Written
Modern Microsoft Office documents are generally a collection of XML files and image files. This collection of files is packaged as a Zip file. LibreOffice can save documents in a Microsoft Office compatible format, but its file format differs in two important details: (1) the GMT time that the file was saved is recorded in the Zip file components that make up the final document and (2) the document internal last saved time is recorded as local time (unlike Microsoft Word, which records it as a GMT [UTC] value).