ESTIM_BIRT_DATE: estimated date of birth, whether:

 - BIRT_DATE
 - 'ABT'|'AFT'|'BEF'|'CAL' DD MMM YYYY -> DD MMM YYYY
 - 'ABT'|'AFT'|'BEF'|'CAL' MMM YYYY    -> 1 MMM YYYY
 - MMM YYYY                            -> 1 MMM YYYY
 - 'ABT'|'AFT'|'BEF'|'CAL' YYYY        -> 1 JUL YYYY
 - YYYY                                -> 1 JUL YYYY

Used by:
 - AGE_MARR_x
 - AGE_AT_LAST_CHILD


FIXME:
 - Handle 'YYYY' (cast BIRT_DATE to string)
 - Use 'CHR_DATE - 30 days' in ESTIM_BIRT_DATE
 - Compute min and max presence (date)