Convert 5 Digit Serial Date Within Access

I am importing a table from a Clarion TPS database which stores several types of data in a "IDVAL" field. The field next to it, "LBLNUM" defines what the data type is. This creates a problem, as IDVAL is a text field, which means all data stored in it (no matter what the type) is then stored as text. This includes date fields. Since they are stored as text, they end up as 5 digit serial dates, similar to how excel handles dates (with each day after 1/1/1900 being +1).

So I have a union query which derives all the values stored in IDVAL for each unique record. My query which creates my export combines this data with several calculated fields to create my final export.

Everything looks great on this final export except for my date fields, which are all 5 digit numbers (like 77945).

I have been searching for a date function that will format this back to a standard date, however I have yet to find anything that works. Is there no way to format a 5 digit serial date back to a standard date within access by using a function? Every answer I find says to redefine the table to a date field, however I don't have this option since the field in question stores a lot more than just dates.