contrary to the documentation for date and decimal-date-time, these 2 functions are not the inverse of each other
date(decimal-date-time(${mydate})) is not always the same as ${mydate}
this is because decimal-date-time() can produce decimals from dates during daylight savings time, but date() only takes integers (thus it casts the decimals to int, flooring them rather than rounding them)
more technically the co-domain of decimal-date-time() is not the domain of date(), therefore they are not the inverse of each other
in practical terms, saying they are the inverse of each other is misleading, and suggests that date() can handle decimals