The default comes from this setting in mojoProfile.config
<add name="TimeZoneId"
type=""
iSettingControlSrc="~/Controls/TimeZoneIdSetting.ascx"
labelResourceKey="TimeOffsetLabel"
defaultValue="Eastern Standard Time"
requiredForRegistration="true"
regexValidationExpression=""
regexValidationErrorResourceKey=""
includeHelpLink="false"
/>
You should use a custom version of this file as documented here instead of just changing it in mojoProfile.config
http://www.mojoportal.com/userprofileconfiguration.aspx
Hope it helps,
Joe