A library and webservice I am using, communicates time-intervals in ISO 8601: PnYnMnDTnHnMnS. I want to convert such formats to seconds. And vice versa. Seconds are a lot easier to calculate with.
Example interval values are:
It looks like PHP 5.3's DateInterval supports this.
If you can't use 5.3, I suppose any such conversion function would know how many seconds are in a year, a month, a day, an hour, and a minute. Then, when converting from seconds, it would divide in that order, each time taking the modulo of the previous operation, until only <60 seconds are left. When converting from an ISO 8601 interval representation it should be trivial to parse the string and multiply each found element accordingly.