Desktop (.desktop) icon with custom environment variables

I need to set an environment variable for an application launched as a .desktop icon. I'm not seeing any reference to variables in the latest version of the specification. So I am trying a work-around with a shell:

Code:[Desktop Entry]
Name=Oclock
GenericName=Clock
Comment=Display Time
Type=Application
Exec=sh -c "TZ=America/Chicago oclock -transparent -geometry 94x94+110+0"
Icon=/usr/share/icons/Mint-X/apps/96/clock.svg
StartupNotify=false
Terminal=false
Categories=Clock;Would there be a relevant desktop entry key, or is the above method the only way?