Andrew,

The pattern has to match exactly

d = DateObject[{"06/11/2019 10:55:44 AM", {"Month", "/", "Day", "/", 
    "Year", " ", "Hour12", ":", "Minute", ":", "Second", " ", "AMPM"}}];
DateString[d, "ISODateTime"]
(* 2019-06-11T10:55:44 *)

Change AM to PM

d = DateObject[{"06/11/2019 10:55:44 PM", {"Month", "/", "Day", "/", 
    "Year", " ", "Hour12", ":", "Minute", ":", "Second", " ", 
    "AMPM"}}];
DateString[d, "ISODateTime"]
(* 2019-06-11T22:55:44 *)

Hi Andrew,

To parse a date string in M/D/Y format:

d = DateObject[{"6/10/2019", {"Month", "/", "Day", "/", "Year"}}]
DateString[d, "ISODateTime"]
(* 2019-06-10T00:00:00 *)