Calculate time difference

I have time in a file in HH:MM:SS format as it contents(its not the file creation time). i need this to be converted to epoch time or time since 1970. The time is written into that file by a script, which i cannot modify. Im using AIX machine

$ cat abc.txt
10:29:34

I might be missing something here...

How can you get epoch time from just 'HH:MM:SS'?

What day, month and year would the file contents be refering to?

sorry it has date and time in it

cat file.txt
16/02/21 10:29:34