 <p>Converting is not an option, since you don't know anything about what the user meant. For example, when I input the number '3', you can't determine if I meant 3 days or 3 months.</p> <p>Let me elaborate a bit: if I input '3' and select 'days', the time in seconds is: 60 * 60 * 24 * 3 = 259200 seconds. When displaying, you could divide it so that the output is '3 days' again. But what if I inputted '72' and selected 'hours'? You can't tell.</p> <p>Just store the users choice and you're fine.</p>