DateTime represents a defined Date and Time which could be in the past, present or future.
The structure of this datatype is internally considered a Struct, and as such has some unusual properties
[Microsoft's Documentation] on DateTime has more on this data type.
The data type does not directly convert to Milliseconds without the conversion algorithm. DateTime even has data about time zone and daylight savings time from the time zone it was recorded in within its value, as well as versioning. As such, working directly with the ULong that can sometimes be extracted from it is not recommended.