Subtotals and Grand Totals are two very useful tools to keep groups of data aligned as going from granular row level data for potentially many rows to a single Grand Total can be disorienting.
Insight can produce Subtotals and Grand Totals on tabular reports where numerical data is displayed. Insight cannot take non-numeric data, such as timestamps or time formatted strings and perform aggregation on them, such as creating a summed Subtotal or Grand Total over groups. This limitation is due to the fact that Insight does not include a parser and interpreter of such non-numeric formats to convert them into a numeric format then present them back in their original format.
Subtotals and Grand Totals can be performed on non-numeric columns, such as those relating to talking time if the column(s) are presented to Insight as a number. That is, instead of trying to work with row data consisting of values such as "00:04:23" Insight would instead be presented with a value of 4.383 (converted to minutes) or 263 (converted to seconds). The catch is the Subtotals and Grand Totals would also be in the same numeric format.
While there is nothing wrong with working in numeric formats, some individuals don't prefer them. As such, we do not perform any Subtotal or Grand Total calculations out of box on reports in all places where they may make sense. With your feedback, we may include them as numeric representations of a time precision like minutes or seconds in later updates.