Formatting time between Rails and Javascript -- avoiding duplicating code across languages

I have a view that takes a Ruby Date, performs some formatting on it to output just the time in the format [5:30 P开发者_StackOverflow中文版M]. Then by default it adds 30 minutes to that so the result is similar to the following output:

5:30 PM - 6:00 PM

and the code in the haml file looks like:

=format_time( @time ) + " - " + format_time( Time.parse( @time ).advance( :minutes => 30 ) )

where format_time is a method I created.

But then the user can change the 'duration' by changing a combobox selection. Let's say the user chooses '60' in the combobox, I need to the add 60 minutes to the start time and update the output to read:

5:30 PM - 6:30 PM

The problem is that when the page first loads, the formatting happens in my Ruby method format_time. But after the combobox selection I think the formatting has to be done in Javascript triggered by the 'onchange' event on the combobox. But that means I have to maintain two different methods to do the same formatting.

Is there a better solution to this?

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One option is the have the javascript send out an AJAX request to some rails page whose sole job is to format the date using the same method as before.

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Do all the formatting in javascript. Have the haml output the unformatted time as a javascript variable, then format and display it with javascript. That will avoid extra AJAX calls and let you keep the formatting code in exactly one place.