Filter using Date Time with UTC offset

[stevenamani]

I am create a filter in JavaScript as follows:

target: {table: "Encounter",column: "AdmitDate"},logicalOperator:"Or",conditions: [{operator: "GreaterThanOrEqual",value:"2015-04-04T00:00:00.300Z"}

The above works fine and I assume gives me 3 hours from UTC. However, when I try to use a negative value, it errors with InvalidFilter:

target: {table: "Encounter",column: "AdmitDate"},logicalOperator:"Or",conditions: [{operator: "GreaterThanOrEqual",value:"2015-04-04T00:00:00-300Z"}

what is the correct for -X hours from UTC?