I ran across this issue today and wanted to share the solution. If all your MOSS 2007 calendars look like this, showing the dates twice:
Go to your portal home page, choose Site Actions / Site Settings / Modify All Site Settings (if you have rights to do so), then choose Regional Settings under the Site Administration heading. Make sure Alternate Calendar is set to "None," and then click the "All sub-sites inherit these Regional Settings" box at the bottom. Click OK, and your calendars should show only one date heading.
Of course, you could always have one show Gregorian and one show Japanese Emperor Era, your call.