Understanding Society User Support

Hi, my current research requires me to create a measure of proportion of household joblessness a respondent experienced during 1991-2000 when co-residing in their parents' household and a corresponding measure of respondent joblessness for the periods of 2008 onwards. Specifically, I have the following urgent question:
1) For Understanding Society, this particular thread (https://www.understandingsociety.ac.uk/support/issues/308) made me realise that the data structure for BHPS and Understanding Society is slightly different. Specifically, Understanding Society does not provide spell starts, only spell ends (which is not the case for BHPS for which I used Mare's code). I understand that we can just set the spell start equal to the spell end of the previous activity spell for most cases for Understanding Society. However, if we have cases where an Understanding Society sample member who were part of the original BHPS sample did not provide valid information on the all the employment/non-employment spells they experienced until wave 3 or onwards in Understanding Society, there is the problem of a gap in the information and the missing start date of the first employment/non-employment the person reported for Understanding Society is effectively unknown. What should I do about such a situation?