I have log from a tool in this format: hh:mm:ss.uuu.
For example: 10:12:34.323
Setting these cells in Excel 2003 to time format with syntax hh:mm:ss.000 is accepted, but any operation, like add/subtract, yelds to #Value! error.
Is there any way for Excel to do calculation on already set custom and recognized time format, without extra effort?
Update with screenshots:

