Finding the date/time a file was first added to a Git repository

git log --follow --format=%ad --date default <FILE> | tail -1

With this command you can out all date about this file and extract the last

The option %ad shows the date in the format specified by the –date setting, one of relative, local, iso, iso-strict, rfc, short, raw, human, unix, format:<strftime-string>, default.