When Cheech and David walk through a neighborhood after the Boston opening, the corner has an obvious modern accessibility curb-cut.
There are two posters for The Wild Party (1929) outside the Rivoli. However, the relevant scene takes place in September 1928 and that film was not released until April 1929.
When David stands in the street and argues with Sheldon Flender in the apartment above, a powerful floodlight on a technical-looking stand is reflected in the open window.
Helen mentions that she hasn't had a drink since New Year's Eve and clarifies that she means Chinese New Year. "Still," she says, "that's two days." The film begins at the end of September 1928. Chinese New Year was on January 22 of that year.
Helen gives David a gift on his birthday, and calls him a Scorpio. Two scenes later, it is September 24th. The sun enters the sign of Scorpio at the end of October. David is a Virgo or a Libra.