Formatted value of a Calc date cell?

I'm having trouble understanding how to get the formatted value of a date
cell. I'm using the PyUNO binding of the API, and trying to reformat a date
cell as YYYY-MM-DD. Here's (a somewhat slimmed down version of) the relevant
portion of the code, with various things I've tried in comments.

    locale = document.getPropertyValue("CharLocale")
    formats = document.getNumberFormats()
    formatID = formats.queryKey("YYYY-MM-DD", locale, False)
    line = []

    for r in range(dataRange.StartRow, dataRange.EndRow):
        for c in range(dataRange.StartColumn, dataRange.EndColumn):
            cell = sheet.getCellByPosition(c,r)
            cellType = cell.getType()

            if cellType == UNO_VALUE and cell.NumberFormat == UNO_DATE:
                cell.NumberFormat = formatID
                line.append(cell.getText().String)
                # line.append(cell.String)
                # line.append(cell.getString())
                # line.append(cell.Value)
                # line.append(cell.getValue())
                # line.append(cell.Formula)

All of the above give me floating point numbers instead of a date. I've also
tried displaying the value without changing the format. What am I missing?