Date a document when it is written, not when it is opened

[jafin]

Fixes #365.

The problem

PdfReader.Open sets the modification date to the current time as part of opening a document for modification, before anything has been modified and whether or not the document is ever written:

// Change modification date
document.Info.ModificationDate = DateTime.Now;

PdfSharpCore/Pdf.IO/PdfReader.cs, in the openmode == PdfDocumentOpenMode.Modify block.

Opening a file and touching nothing else is enough to date it as edited:

opened ReadOnly  CreationDate = 2026-07-28 02:49:06   ModDate = <unset>
opened Import    CreationDate = 2026-07-28 02:49:06   ModDate = <unset>
opened Modify    CreationDate = 2026-07-28 02:49:06   ModDate = 2026-07-28 12:49:06   <- now

That leaves no way to tell a document that has been changed from one that has only been read, which is the one thing a modification date is for. The reporter of #365 worked around it by opening read only.

The change

The stamp moves to PdfDocument.PrepareForSave, where the creator and the producer are already filled in. The condition is the one that was there before, evaluated at save time: a document PdfReader opened for modification. IsImported is part of it because Modify is the zero value of PdfDocumentOpenMode, so a newly created document would otherwise match it.

Two details:

• A date the caller chose is left alone. Callers can override the date today, because opening stamps first and saving does not. PdfDocumentInformation.ModificationDate now records that it was set from outside, and the stamp skips it.
• The stamp is written through the element, not the property, so the first save's stamp is not mistaken for a caller's choice when the next save comes around. Covered by its own test.

A document written from scratch is unaffected: dated by its creation date alone, carrying no /ModDate, as before.

Behaviour change to be aware of

Code that reads Info.ModificationDate straight after PdfReader.Open(..., PdfDocumentOpenMode.Modify) and expects a value now gets whatever the file says, which for a document with no /ModDate is DateTime.MinValue. That is the fix rather than a side effect, but it is a visible change and worth knowing before merging.

Tests

PdfSharpCore.Test/IO/ModificationDateTests.cs, six tests: what opening reports, that an undated document stays undated, that writing stamps, that writing twice stamps twice, that a caller's date survives a save, and that an authored document is not stamped. Four of them fail without the change.

The test for what opening reports compares it against opening the same file read only, rather than against a literal date, which keeps it clear of how a date is spelled in a file and what time zone it comes back in.

The tests build their documents in memory and draw nothing, so they need no font resolver and no assets.

Note on the existing test suite

Five XTextFormatterTest cases fail on my machine, and they fail the same way on an unmodified checkout of master, so they are not from this change — they compare rendered pages against PNG baselines and are sensitive to the fonts installed. They may well be green on the Linux CI runner the baselines came from.

[Copilot]

Pull request overview

This PR fixes an observed metadata side-effect where opening a PDF in Modify mode immediately updated /ModDate, even if the document was never saved. It moves modification-date stamping to save time and adds regression tests to ensure /ModDate reflects actual writes while preserving caller-specified dates.

Changes:

• Move modification-date stamping from PdfReader.Open(..., Modify) to PdfDocument.PrepareForSave().
• Track whether the modification date was explicitly set by the caller to avoid overwriting it on save.
• Add a dedicated test suite covering open/save behaviors for /ModDate.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
PdfSharpCore/Pdf/PdfDocumentInformation.cs	Adds a flag to detect caller-set modification dates so save-time stamping can skip them.
PdfSharpCore/Pdf/PdfDocument.cs	Stamps /ModDate during PrepareForSave only for imported documents opened in Modify mode (unless caller-set).
PdfSharpCore/Pdf.IO/PdfReader.cs	Removes open-time /ModDate stamping to prevent read-only-like operations from mutating metadata.
PdfSharpCore.Test/IO/ModificationDateTests.cs	Adds regression tests validating open-time reporting and save-time stamping behavior.

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