In the time-dependent society of today, those with limited ability to manage time will show a heightened dependence on others and more need for support, exacerbating their vulnerability. Children with developmental disorders e.g. intellectual disabilities or Autism Spectrum Disorders are reported to have problems in time perception, time orientation or time management, in this chapter all related to the suggested overarching concept: time processing ability (TPA). The aim of this chapter is to present a currently developed innovative assessment of TPA and its relation to daily time management (TM) in children with and without developmental disabilities. The new assessment of TPA; Kit for assessing Time processing ability (KaTid) for children was based on current knowledge of information processing in children with cognitive disabilities e.g. using iconic symbols and concrete concepts. Using modern test statistics (Rasch analysis), the evaluation of the KaTid and a related Parent scale for estimating the daily TM presented acceptable psychometric properties. The results indicate that the items in KaTid, initially defined as time perception, time orientation and time management, all empirically support a potential uni-dimensional construct, TPA. In this TPA construct, time perception, time orientation and time management can be seen as different levels of complexity rather than as separate constructs. The Parent scale of daily TM also demonstrated acceptable psychometric properties. There is a relation between the parents' ratings of a child's daily TM and TPA in children with developmental disabilities. Thus TPA seems to be a factor related to children's daily TM that needs to be taken into consideration when planning and evaluating interventions designed to facilitate everyday functioning for children with ID. This is a step towards providing evidence-based instruments that, together with other information, can help the professional in structuring the intervention planning process for children with difficulties in daily TM.