I want to pass it the current time, ideally through a shell script that I can execute when the computer boots up.
Were the Mac up and running with the Leonardo connected, the Arduino would be a virtual com port by virtue of the iOS USB driver... Then you could simply script the current time into a variable and echo that variable as a redirect to the virtual com port. In Windows, the ECHO command could simply be used with a redirect to the virtual com port.
However, at boot-up time, I am unsure if iOS would have enumerated the USB to a virtual com port and I am unsure how you can configure iOS to always assign the same virtual com port number; therefore the script may need some intelligence to integrate the environment while waiting until all USB enumerations are complete. Assuming the Arduini is always the same serial port number, then you would simply redirect an echo of time-date to the appropriate comport.