Download of NengoCalc 4.1 for Mac was available from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The application lies within Productivity Tools, more precisely Calculators. We recommend checking the downloaded files with any free antivirus. The actual developer of this free software for Mac is SilentMountain.
From the developer:
With this tool, the user can convert Japanese dates into Western equivalents. NengoCalc uses a comprehensive database of chronologcial data and era names to accomplish the calculation of the Western dates. The main algorithms and tables are derived from Tsuchihashi.
You may want to check out more Mac applications, such as CalaMatic Scientific Calculator or UltiCalc 3, which might be similar to NengoCalc.