Don't use isinstance for simple datetimes.
Don't use isinstance for simple datetimes.

Significant increase in performance for this one.

      rows       cols    elapsed       size
       144        144       0.65     149608
       144        233       1.04     158128
       144        377        1.7     208816
       233        144       1.15     149632
       233        233       1.83     158128
       233        377       2.89     208864
       377        144       1.78     149680
       377        233       2.98     158104
       377        377       4.68     208840

jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter,jmcnamara/XlsxWriter