To get the value as a string, check this example:
declare @json varchar(max) = '
{"i:0#.w|my\\jeffw":{"DateTime":"\/Date(1604906464292)\/", "LoginName":"SHAREPOINT\\system"},
"i:0#.w|my\\yex":{"DateTime":"\/Date(1604909155060)\/", "LoginName":"SHAREPOINT\\system"},
"i:0#.w|my\\danj":{"DateTime":"\/Date(1604914548113)\/", "LoginName":"my\\jeffw"}}
'
declare @result varchar(max)
set @result = json_value(@json, '$."i:0#.w|my\\danj"."DateTime"')
select @result
You can also access it by LoginName, if it is unique. It depends on how you identify the data.
A similar approach can be used with database columns.
(It is possible to convert this result to datetime, which is probably a different question).