Calculating duration with strange date / time format

I had a simple formula for working out a duration based on 2 columns of data. Each column had a date and a time in it in this format in it: "dd/mm/yyy hh:mm"

The formula was this: "(F2-E2)*24*60*60)"

Unfortunately now the date and time format has changed to be like this:
"[01:02:2013][04:06:05:062]" so basicly there are now "[]" around the date and time and the time now includes seconds and milliseconds as well.

The formula now longer works and I have tried to create a custom number format in the hope it will recognise the new format, but I cant get it work (either to accept the new format or read the date/time correctly)

This is what i tried
"["dd:mm:yyyy"]""["hh:mm:ss:000"]"

Does any one have any ideas how this can be achieved please? (The milliseconds are that important)

Thank you in advance for any help!