Hmmmm. I still smell a rat.
The timestamp Plex uses is the timestamp of the file
From that point forward, internally it’s relative to “Date Added”
When matched, it also stores info about when the episode / movie were “first available at”
I purchased this test video disk and added them in 2022.
It shows in the XML.
<MediaContainer size="1" allowSync="1" identifier="com.plexapp.plugins.library" librarySectionID="1" librarySectionTitle="Testfiles" librarySectionUUID="93da284f-27ad-4246-8621-7ff86964cb32" mediaTagPrefix="/system/bundle/media/flags/" mediaTagVersion="1711462660">
<Video ratingKey="56" key="/library/metadata/56" guid="com.plexapp.agents.none://f5c1a5b31c5b053328c03935044026b5d1dc1ff0?lang=xn" type="movie" title="Spears & Munsil UHD HDR Benchmark_t31" librarySectionTitle="Testfiles" librarySectionID="1" librarySectionKey="/library/sections/1" subtype="clip" summary="" year="2022" thumb="/library/metadata/56/thumb/1683526607" art="/library/metadata/56/art/1683526607" duration="300320" originallyAvailableAt="2022-11-16" addedAt="1668654963" updatedAt="1683526607" createdAtAccuracy="epoch,local" createdAtTZOffset="0">
<Media id="56" duration="300320" bitrate="4791" width="3840" height="2160" aspectRatio="1.78" audioChannels="6" audioCodec="truehd" videoCodec="hevc" videoResolution="4k" container="mkv" videoFrameRate="24p" videoProfile="main 10">
<Part accessible="1" exists="1" id="56" key="/library/parts/56/1668654963/file.mkv" duration="300320" file="/vol/media/testfiles/Spears & Munsil UHD HDR Benchmark_t31.mkv" size="180395152" container="mkv" deepAnalysisVersion="6" requiredBandwidths="4715,4715,4715,4715,4715,4715,4715,4715" videoProfile="main 10">
<Stream id="205" streamType="1" codec="hevc" index="0" bitrate="761" language="English" languageTag="en" languageCode="eng" bitDepth="10" chromaLocation="topleft" chromaSubsampling="4:2:0" codedHeight="2160" codedWidth="3840" colorPrimaries="bt2020" colorRange="tv" colorSpace="bt2020nc" colorTrc="smpte2084" frameRate="23.976" height="2160" level="153" profile="main 10" refFrames="1" requiredBandwidths="750,750,750,750,750,750,750,750" width="3840" displayTitle="4K HDR10 (HEVC Main 10)" extendedDisplayTitle="4K HDR10 (HEVC Main 10)"> </Stream>
<Stream id="206" streamType="2" selected="1" default="1" codec="truehd" index="1" channels="6" bitrate="317" language="English" languageTag="en" languageCode="eng" audioChannelLayout="5.1(side)" bitDepth="24" requiredBandwidths="313,313,313,313,313,313,313,313" samplingRate="48000" title="Surround 5.1" displayTitle="English (TRUEHD 5.1)" extendedDisplayTitle="Surround 5.1 (English TRUEHD)"> </Stream>
<Stream id="207" streamType="2" codec="ac3" index="2" channels="6" bitrate="640" language="English" languageTag="en" languageCode="eng" audioChannelLayout="5.1(side)" requiredBandwidths="630,630,630,630,630,630,630,630" samplingRate="48000" title="Surround 5.1" displayTitle="English (AC3 5.1)" extendedDisplayTitle="Surround 5.1 (English AC3)"> </Stream>
<Stream id="208" streamType="2" codec="dca" index="3" channels="6" bitrate="1564" language="English" languageTag="en" languageCode="eng" audioChannelLayout="5.1(side)" bitDepth="24" profile="ma" requiredBandwidths="1540,1540,1540,1540,1540,1540,1540,1540" samplingRate="48000" title="Surround 5.1" displayTitle="English (DTS-HD MA 5.1)" extendedDisplayTitle="Surround 5.1 (English DTS-HD MA)"> </Stream>
<Stream id="209" streamType="2" codec="dca" index="4" channels="6" bitrate="1509" language="English" languageTag="en" languageCode="eng" audioChannelLayout="5.1(side)" bitDepth="24" profile="dts" requiredBandwidths="1485,1485,1485,1485,1485,1485,1485,1485" samplingRate="48000" title="Surround 5.1" displayTitle="English (DTS 5.1)" extendedDisplayTitle="Surround 5.1 (English DTS)"> </Stream>
</Part>
</Media>
<Extras size="0"> </Extras>
<Related> </Related>
</Video>
</MediaContainer>
originallyAvailableAt=“2022-11-16” addedAt=“1668654963” updatedAt="1683526607
Go look at the XML for a couple of your files and see what you get ?