Launch windows for Venus transit; how to convert heliocentric longitudes to date?
Heliocentric longitude is a direction, not a date.
A Heliocentric longitude of 0 is the direction from the sun towards the vernal equinox (currently in Pices)
You can ask for the date on which the Earth is at a particular Heliocentric longitude.  A careful calculation would require accounting for the eccentricity of the Earth's orbit, and also adjusting for the oddities of the calendar, but since when the Sun in on the vernal equinox (About March 21), the Earth will be at 180 degrees of heliocentric longitude, and longitude will increase by 360/365.24 degrees per day.