Pattern of saved filename can be configured in Spectacle configuration in "Save" tab as e.g. "Screenshot_%Y%M%D_%H%m%S".
Currently %Y, %M, %D, %H, %m, %S are calculated at the time of pressing "Save As" button and opening "Save File" window or at the time of pressing "Save" button.
The reasonable behavior would be to calculate %Y, %M, %D, %H, %m, %S at the time of capturing screenshot.
Makes sense, an can easily be confirmed by taking a fullscreen screenshot with displaying of seconds enabled for the clock widget.
BTW, let us know in case you are interested in submitting a patch for this (which is the fastest way to get the issue fixed), we are happy to help you getting started.
I created diff D15106 to fix this bug.
Git commit 006bbaa0f63c764b04451f938afe39f7afa0cc18 by Nathaniel Graham, on behalf of Kyle Utecht.
Committed on 11/09/2018 at 12:42.
Pushed by ngraham into branch 'master'.
Calculate date and time values for file name at time of screen shot rather than time of save
Added property to ExportManager to hold timestamp of screenshot that is then used when creating the file name instead of the file's save timestamp.
Reviewers: #spectacle, ngraham
Reviewed By: #spectacle, ngraham
Differential Revision: https://phabricator.kde.org/D15106
M +6 -1 src/ExportManager.cpp
M +3 -0 src/ExportManager.h
M +1 -0 src/SpectacleCore.cpp