If you want something simpler:

    now() {
      echo $(date "+%Y-%m-%d %H:%M:%S") - "$@" >> $HOME/.now
    }

Usage:

    now "Working on Project A"
    now pause
    now continue
    now done

You won't get a nice report and you'll have to calculate hours yourself, but it supports every edge case you can think of. Need to back-date an entry? Just open the file and change the timestamp. Want to nest tasks? Just do `now $ working on sub task` and you can easily see the indentation.