Hi, as the conversion between concrete dates could be expensive I guess that these conversion must be explicit. But this has some consequences when used the implicit conversion was hidden a not efficient implementation, e.g. date ISO_week_start = mon <= jan/day(4)/y; jan/day(4)/y should be ymd_date as is the efficient representation. The date generator was declared as date operator<=(weekday wd, date x); but it works efficiently only for days_date. If we provide only the functions that are efficient we should declare it as days_date operator<=(weekday wd, days_date x); So the preceding expression would need an explicit conversion days_date ISO_week_start = mon <= days_date(jan/day(4)/y); Do we want to go on this direction? A radical alternative to the explicit construction, if we want to make evident that the conversion operation could be expensive, is to use a compute_ factory days_date ISO_week_start = mon <= compute_days_date(jan/day(4)/y); Best, Vicente P.S. the generator function is just an example needing explicit conversion.