That is not a proper date format!
ObjectCreate(0,"arrow",OBJ_ARROW,0,2018-09-20,1.3297);
This is a proper date format:
ObjectCreate(0,"arrow",OBJ_ARROW,0,D'2018.09.20',1.3297);
Read the documentation on the datetime data type:
Datetime Type
The datetime type is intended for storing the date and time as the number of seconds elapsed since January 01, 1970. This type occupies 8 bytes of memory.
Constants of the date and time can be represented as a literal string, which consists of 6 parts showing the numerical value of the year, month, day (or day, month, year), hours, minutes and seconds. The constant is enclosed in single quotation marks and starts with the D character.
Values range from 1 January, 1970 to 31 December, 3000. Either date (year , month, day) or time (hours, minutes, seconds), or all together can be omitted.
With literal date specification, it is desirable that you specify year, month and day. Otherwise the compiler returns a warning about an incomplete entry.
Examples:
datetime NY=D'2015.01.01 00:00'; // Time of beginning of year 2015 datetime d1=D'1980.07.19 12:30:27'; // Year Month Day Hours Minutes Seconds datetime d2=D'19.07.1980 12:30:27'; // Equal to D'1980.07.19 12:30:27'; datetime d3=D'19.07.1980 12'; // Equal to D'1980.07.19 12:00:00' datetime d4=D'01.01.2004'; // Equal to D'01.01.2004 00:00:00' datetime compilation_date=__DATE__; // Compilation date datetime compilation_date_time=__DATETIME__; // Compilation date and time datetime compilation_time=__DATETIME__-__DATE__;// Compilation time //--- Examples of declarations after which compiler warnings will be returned datetime warning1=D'12:30:27'; // Equal to D'[date of compilation] 12:30:27' datetime warning2=D''; // Equal to __DATETIME__The string representation of datetime type depends on compilation mode:
datetime date=D'2014.03.05 15:46:58'; string str="mydate="+date; //--- str="mydate=1394034418" - without #property strict //--- str="mydate=2014.03.05 15:46:58" - with #property strict

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According to the documentation on the ObjectCreate function (https://docs.mql4.com/objects/objectcreate), I should be able to pass a datetime value for the anchor point. I have used multiple formats for the datetime value. Regardless of the format of the datetime value, the object is always placed at 1970.01.01 00:00