Generate ctags incrementally in vim. Juste paste this into your .vimrc.

incremental_tags.vim

function! GenerateTagsIncrementally()
python << EOF
import os
print('Generating tags incrementally.')
# Walk the file tree, if a file has an mtime more recent than the tag file,
# add it to the list of files to index.
tags_mtime = os.stat('tags').st_mtime
with open('list', 'w') as fp:
    for dirpath, dirnames, filenames in os.walk(os.getcwd()):
        for filename in filenames:
            full_path = os.path.join(dirpath, filename)
            if os.stat(full_path).st_mtime > tags_mtime:
                fp.write(full_path + '\n')
                # print(full_path) # Files to be indexed.

# Run ctags using the created list of files.
os.system('ctags --recurse --verbose --append --extra=+q --fields=+aimS --c-kinds=+p --c++-kinds=+p -L list')
os.remove('list')
EOF
endfunction
command GenerateTagsIncrementally call GenerateTagsIncrementally()

I think the magic you use is just the --append option.

You can just use the find command to find all the file modified after the tag file like this:

find . -newer tags #tags is the name of you tag file

However, the option --append does not work well when you delete some code from your source file. Even though a symbol is deleted from the source file, the --append option could not let cags to delete the symbol from the tag file.

line 18:
Traceback (most recent call last):
File "", line 5, in no
OSError: [Errno 2] No such file or directory: 'tags'