The time decay attribution model is a conversion attribution model by which the conversion value is assigned to all the campaigns, mediums or keywords that have participated generating the conversion, proportionally, but the system gives more value if the click is nearer of the conversion.
The time decay attribution model is not used by default in almost all the Web Analytics Tools, like Google Analytics. Normally the Analytics Solutions have a multi-touch attribution modeling reports, where you can compare different attribution models.
The time decay model is appropriate for conversions which are not impulse buying without a consideration phase. This attribution model is one of the best who work with because assign to all the participants in getting the conversion a proportion of the conversion and it gives more importance if the click is close to the conversion.
If you are working in a very competitive market, where the decision is not taken easily and where the users have to do more than one click to make the conversion, linear Attribution is one of the best attribution models for your business.