I use the very respectable SlimTimer to help me track my hours. Unfortunately, while they display a consistent total across all reports, the entries in each report do not necessarily add up to that total due to the fractional time units and the rounding involved.
Unfortunately, the accounting department tends to frown upon inconsistencies like this, no matter the reason. My process thus far has been to simply export a full timesheet with the report settings specifying time units as precisely as possible, and then performing the sum myself on the resulting chart.
This got a bit tedious, so I wrote a program to compile the necessary tallies over however many timesheet files I wanted to process. The source and binaries are available for download. Simply drag and drop any number of timesheets generated from SlimTimer, and the utility will generate a new csv file for each timesheet, with an extension "-out.csv".
The format of the output csv file is formatted for my invoice structure, but adjusting it for other formats is fairly trivial for anyone familiar with C#.