AFAICT from the WMR88A manual, it only shows rain in the last hour or the last 24 hours. At least according to the manual, it does not display a total accumulated rainfall. If your unit shows something other than last hour or last 24 hours then the manual is misleading or wrong.
Either way, I'm about 90% certain that it will not reset automatically on any given date; there is no consistent world standard for a month/day when the rain year begins.
The WMR100 displays accumulated rainfall and the manual tells you how to reset the total. I don't see any similar information in the WMR88A manual.
Sorry I couldn't offer more help but I don't have a WMR88A...just WMR100.