Quote:
Originally Posted by theducks
There is. sort of 
In the meta-data viewer:
1) If you hover over ant format, it now shows the file date a)Last Viewed in Calibre's viewer (Calibre leave a custom marker inside), (  for EPUB Use T (tweak-epub to explode the epub and see the file timestamps of the pieces inside. you can discard/cancel your Tweak session when done)
2) what formats exist (this assumes the reason to convert was LACK of the desired format  )
What none of this shows: was what format YOU used (master) to convert to others ( or if you even did do a convert in order to get other formats) 
I assume the reason you are 'converting' is that you want Format X
set a search or make a filter: Now your list shows the candidates for conversion TO...
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By metadata viewer I presume you mean the edit metadata window. In the upper right-hand corner of that window it shows the available formats. If I hover over a format, a last modified date is displayed.
I have a Kindle and convert the books from whatever format to mobi, so I won't be using the T option, at least I don't forsee a use for it.