Time estimate mode -plugin

Is it possible to modify the .schedule mode and create a .estimate mode?
It would accept the following inputs: Hours, Half hours I.E. ‘6,5 hours’, Minutes
Instead of showing timestamps in the left margin it would show a tally of total hours (similar to what it does now but displayed as Hours:Minutes I.E. 120:45)
And Instead of ‘Ends’ in the bottom it would Say ‘Total: XXX hours, XXX minutes’ and if no minutes have been input it would just say ‘Total: XXX hours’.

How would I go about it?
Any help is greatly appreciated,