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 Summary: 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. FIXED-IN: 18.12.0 Reviewers: #spectacle, ngraham Reviewed By: #spectacle, ngraham Subscribers: 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 https://commits.kde.org/spectacle/006bbaa0f63c764b04451f938afe39f7afa0cc18