There are still some erroneous ideas that Daoism philosophy includes or is described by  yin/yang theory & that too familiar
Tai Chi` symbol. (Please read for a historical perspective). 
This is quite simply out-of-date non-academic information. It was the interpretations of early populist writers back in the 1960-70’s (80´s).

Scholarly & Daoist writings since the 90’s (or even Feng Youlan’s a History of Chinese Philosophy, written 1934; but in English first  1983 ) clearly point out that Yin-Yang (& the related Wu Xing 5 Element philosophy) come much later in Chinese History as concepts developed during the Warring States Era (475 – 403 BCE) and then further explored by the School of Naturalists (see Zhou Yan 305 – 240 BCE) during the period of the Hundred Schools of Thought.

Daoism, by its nature
of being the indigenous spirituality of a peoples, originates from the very beginnings of that culture – more than 5,000 (- 8,000) years back in history.  Also, by nature of origin it would be arising from the experience of a direct awareness of and immersion within the immediate environments – both near and distant.

An experiential, shamanic or transcendental experience; an observation & description of the nature of what is (perceived around one); which gradually becomes a philosophy as consciousness develops through symbols, words, thoughts and then concepts of mind.

The two basic concepts
or themes of this phenomenological philosophy are then implicit as 1) awareness itself  Shen
and then  2)  awareness of self and self with the environment : Tian    Di    Ren 人  Heaven  Earth  Humanity; personified within the body 
as  Xin  心   Heart-mind.

The period of the Hundred Schools of Thought was a time of immense intellectual flourishing along with intensive political and social chaos and strife. During this period, philosophies were not only keenly discussed and dissected, but also formed, formalised and re-organised!! It was during this time that those ideas of the School of Naturalism (Yin-Yang and Wu Xing) were grouped together with, or added onto, the philosophy of the often reclusive, nature-loving Daoists.

The concept of Yin-Yang  is used ,as example, in Chinese Medicine and other studies, such as Cosmology but not really as a philosophy, it does not have the depth or weight of such. It is, rightly, more understood as a concept of dualism and used, as such, as a way to classify and describe. A manner through which to discriminate the qualities of different phenomena through comparison of one to the other. ´Without noting the length of one stick to the other, I am not able to describe one of them as short, the other as long …`.   One can discern attributes through comparison; this is the usage or the extent of the concept of Yin-Yang, as a good principle for summarising and synthesizing, but not much more

The Tai Chi Symbol, oTai Chi Tu is attributed to Daoist writings as late as the Ming Era (1368- 644 CE); with the simplification of the diagram into just two interlocking spirals as late as 1525-1604 (by Lai Zhide, a scholar of that time). Previous to then, it was most usually written in a form similar to the Celtic Triskele. A more alchemical symbol with moving, spiraling patterns as the representation of three forces or vortexes of nature: Heaven, Earth and Human Life. 

It is understandable that such confusions do arise as the earliest recovered texts are those from the Sung Dynasty (960 – 1279 CE) and the more recent Mawangdui Silk Texts , dated from around 168 BCE (but not recovered until as recent as 1973). Chinese texts written by scholars of their era were known as Jíng  經  (or Ching) Classical work or text ( e.g. Tao Te Ching, I Ching etc.);  and were passed down, usually with the original text unchanged from generation to generation. There would however be many footnotes as commentaries and additions from scholars of the each subsequent generations.

One often awaited until a period of extreme political turmoil arose, with the overthrow of a dynasty, and the corresponding support for an opposing philosophical group for the Classic to be rewritten, usually from an opposing viewpoint. This rewriting would often including many of the commentaries or the ideas found within them or were sometimes more substantially rewritten so that the original meaning or authorship was lost, though the original author’s was name retained as a manner of identification and authority of learning.

An example of this is the Nei Ching , the well known Medical Classic. With its origins seen to be as early as the 4 – 3 century BCE, the most recent ´edition` was collated by Wang Bing during the 12 years upto 762 CE, who ´collected the various versions and fragments of the Su Wen and reorganized it into the present eighty-one chapters (treatises) format`. The ´authoritative version` which is used today for translation is from 1053 CE and is based considerably upon that (762 version) version.

However, one notes that those chapters describing the Cosmological forces and their usage within Chinese Medicine (Su Wen Chapters 64 – 74)  were added by Wang Bing during that period in the 8 century CE. The principles used in the discussion (such as Yin-Yang and Wu Xing) had not been formed until the time of the Han Dynasty (206-220 AD), that is after the dating of the writing of the Su Wen (the first book of the Nei Ching).

 In this case, although one might wish to suggest that the usage of these Cosmological principles in Chinese Medical practice is ´ancient` as they belong to the Nei Ching. They are in fact rather ´recent` (in the context of Chinese history) in their formation, evolution and inclusion as a philosophy of understanding and application for divination or medical purposes.