Troubleshooting an Oracle compute overflow problem (not a divisor of zero!)

The problem has occurred in the previous period:

The table has a column number type rec_recordlength (two time interval length/sec)

Some data statistics This field will appear "Calculate the operation Overflow" error, it is a headache, can not find the reason

And then

Today again, and then constantly troubleshooting, troubleshooting, found that there is a data caused by,

Then this data is removed and reinserted.

Then think of the only thing that moved this data is the Rec_recordlength update, the following actions:

Update  set rec_recordlength=86400*(rec_endtime-where rec _c_id=' 178598 ';

Retry this statement, sure enough is this UPDATE statement, resulting in rec_recordlength statistics save "Calculate the operation Overflow";

I can't figure it out ... is 86400* (rec_endtime-rec_starttime) calculated not the value?

All right, add a conversion:

Update  set rec_recordlength=to_number (86400*(rec_endtime-   where rec_c_id=' 178598 '

There's no problem.

Don't know if this is a bug ....

Ps:oracle 11g

Troubleshooting an Oracle compute overflow problem (not a divisor of zero!)