I have a calculated field (called TO) in a pivot

= if(time >= 600,1,0)

where time is a field.

I present it in 'values' with 'sum'. It shows correct values, but the sub-totals are wrong. I can send an example if you tell me where to...

It's not a bug, it's a consequence of the way calculated fields work. You should read your formula as

=IF([subtotal of time in pivot table]>=600,1,0)

To calculate what you want, add a column with formulas to the source data of the pivot table.