A DW without a time/calendar dimension?  Say it ain't so ... stop all the OBIEE effort and remedy that up front ... it is the very first step in building a DW.

The best way to build a time/calendar dimension is to set it up as a domain sourced (non-system sourced) entity in your DW and set its DW key equivalent to a date (that way you can 'back' into the key relationships)

01/01/2018 date value needs to have a key like 20180101 ... then you can convert your fact dates to keys (in extended columns) and be right back to where you started.

THEN build out the dimension for the attributes that make most sense for the use of dates (contexts of week, month, quarter, year) including the start and end dates for those larger constructs of dates (key to making logical hierarchies work).

Add attributes like holiday_flg (Y/N), work_day_flg (Y/N) etc ...

also add the corresponding previous year key for this date (makes a difference if you want to do day-to-day versus date-to-date analysis.  Retail doesn't do date to date as the same date last year might not hit on the same cycle [16-APR-2017 was a Sunday today is a Monday]. 

Also add in the company calendar designations with keys for each of these dates (fiscal calendars built right in makes for easier working later on).