The default 'mindate' value for the $ZDATE(hdate,dformat,monthlist,yearopt,startwin,endwin,mindate,maxdate,erropt,localeopt)
system function call is 0, which represents 1840-12-31. However, you can supply your own 'mindate' argument with negative values down -672045 which does date conversions more distantly into the past.
USER>zwrite $zdate(-672045,3,,,,,-672045)
"0001-01-01"
which is Gregorian date January 1st in the year 0001 CE. (That is the first day, in the first month in the first year of the first century.) I should note that the Gregorian calendar was not in use back in the year 0001 CE but I don't think IRIS has support for Julius Caesar's calendar. Passing a 'mindate' argument value more negative than -672045 will get you an <ILLEGAL VALUE> signal from $ZDATE() because there is some debate about whether the previous year is 0000 CE, -0001 CE or +0001 BCE.
$ZT is the abbreviation for the $ZTRAP system variable. You learn more about it at https://docs.intersystems.com/iris20251/csp/docbook/Doc.View.cls?KEY=RCO...
Setting SET $ZTRAP="^%ETSDK" will call a routine name ^%ETSDK but I have no idea what the routine is. Maybe it is a special Error Trap routine used at your site.
Executing
SET $ZTRAP="^%ETN"
in an ObjectScript routine will setup the ^%ETN utility routine as an error trap routine which will catch an error signal and then dump the process state into the namespace before terminating the process. Later, executing the DO ^%ERN utility will allow you to examine the various error trap dumps in a namespace.
The %Stream.DynamicCharacter (%SDC) and %Stream.DynamicBinary (%SDB) classes are designed to hold a copy of the string data that is in an element of a %DynamicArray (%DA) or a %DynamicObject (%DO) class. The %SDC class, and its %SDB subclass, were designed to be small and efficient even when the string in the %DA/%DO class is very large, containing many billions of characters or bytes. The %SDC classes contain readonly data so you cannot modify the string data that they contain. However, you can create any other object in %Stream package of classes that supports the CopyFrom() method (such the %Stream.TmpCharacter, %Stream.TmpBinary, %Stream.FileCharacter or %Stream.FileBinary classes) and use the CopyFrom() method to create a writeable copy of the string data in the %SDC object. If your %SDC stream contains many billions of data elements then the
DO tmpStream.CopyFrom(sdc)
access will take much longer to load the tmpStream object than it took to create the original sdc object.
A %DA/%DO object internally contains a data structure called a Packed Vector Array (PVA). The PVA supports rapid creation from JSON text and also rapid data lookup. It does not support rapid data modification, although the %Set() method can be used to modify and rearrange the data in a PVA. When a %DA/%DO contains a string element longer than a selected length (currently defaulting to about 4095 characters) then those characters are placed in a separate buffer object and a small object reference (oref) is placed in the PVA. The buffer classes are the %DynamicString, %DynamicBinary and %DynamicBase64 classes which are used to contain the characters and bytes of long strings. These classes are not very interesting to ordinary ObjectScript programers as they have no properties and no methods. When a %SDC object is created (using something like the SET sdc=dao.%Get(key,,"stream") ObjectScript statement) then the DAOData property in the %SDC object references the buffer class so that characters/bytes in the buffer do not need be copied. A buffer class can be shared by multiple %DA, %DO, %SDC and %SDB objects, which is why the buffer objects are read only.