Hi Guruprasad,
how did you create the model (LDM)? Have you scanned your BigQuery and generated LDM or have you created LDM and mapped it to PDM manually?
When you create Date dataset manually, by default TIME granularities are not checked. Even when you map DATE reference to TIMESTAMP column, granularities are not regenerated. You have to check TIME granularities manually in this case.
Here is a screenshot of default granularities in DATE dataset created manually: