Submit Date in My Requests module is based on GMT. Identity Portal doesn't honor any localization configuration.
For example, when en_AU (English Australia) localization is configured, if a user submit a request before 11.00AM AEDT (Melbourne time) in the morning then the My Requests screen shows the submitted date as yesterday, if submission happened after 11.00AM AEDT then submitted date is shown as today.
11:00 AM AEDT equals to 12:00 AM GMT.