Currently, the tooltip for build time construction is only valid for units that have build rate = 1. For example, a worker elephant (build rate = 2) starting a barracks will see the message "This foundation will be completed in 150 seconds", but he will actually finish it in half the time. Also, currently the formula uses an integer percentage for the building progress rather than the actual value, which makes the countdown jerky rather than smooth. For example, instead of going 10, 9, 8, it'll stay at 10 for two seconds before jumping to 8.
To correct this we need to keep track of the building rates of every builder: for example, women will have rate 1 while men with female inspiration with have rate 1.1, and I believe some heroes affect everyone's building rate. When we add a worker to the foundation we start them with rate = 1, and after their first second of work we update their rate to the actual value. At any time later if their rate changes (for example, if a woman walks past a man and adds +10% to his build rate), we update the stored value.
The total build rate is the sum of their rates, although with multiple builders there is an additional penalty. For example, ten women build at a rate of 10^0.7 = 5.01 instead of 10, half the speed. (That's why building with one unit is the most efficient.)
There are actually two tooltips, "time to completion" and "time speed-up" (which means how much quicker we'd finish the construction if we added another woman builder). I moved both calculations to Foundation.js and corrected the math by using the total build rate rather than number of builders (the two are equal only if every unit has rate = 1).