Photo Tools XP is an app including several features for photographers.
This first version includes a tool for calculating exposure time when you are using ND (Neutral Density) filters without the need to do it manually.
You just need to set the initial exposure time (set your camera to aperture priority mode, check the exposure time and input it into the app) and then you can select the desired filter and/or alter other settings such as aperture and ISO. The global exposure time is returned.