Just saw someone posted something on FB that today is guru rinpoche day....
I know some centers in Chinese culture area (eg. Taiwan) using Chinese lunar calendar, which sometimes have one day different with Tibetan lunar calendar (they often have intercalary month different, but that doesn't influence the issue here). I think it's ok, because all these 'dates' are not that astronomically precise anyway.
But I can't really be convinced of using solar calendar for tsok days... because in the original text, 10th and 25th were described as '10th or waxing moon' and '10th of waning moon'.... If there's any reason people should do certain things around certain moon phase, I don't think the reason is the 'number' of the date.
I mean, it's fine if people want to just choose a day for convenience, but then it can be any day really.
do you know which western centre using solar calendar for practice and what's their explanation of using it?