Going by Zodiac sign meanings I think Smithers is a Pisces. so his birth day would be from February 19 to March 20.

Pisces - The Fish
February 19 – March 20
Also unassuming, the Pisces zodiac signs and meanings deal with acquiring vast amounts of knowledge, but you would never know it. They keep an extremely low profile compared to others in the zodiac. They are honest, unselfish, trustworthy and often have quiet dispositions. They can be overcautious and sometimes gullible. These qualities can cause the Pisces to be taken advantage of, which is unfortunate as this sign is beautifully gentle, and generous. In the end, however, the Pisces is often the victor of ill circumstance because of his/her intense determination. They become passionately devoted to a cause – particularly if they are championing for friends or family.



Mouth Traits:

FEBRUARY
Abstract thoughts
Loves reality and abstract
Intelligent and clever
Changing personality
Temperamental
Quiet, shy and humble
Low self esteem
Honest and loyal
Determined to reach goals
Loves freedom
Rebellious when restricted
Loves aggressiveness
Too sensitive and easily hurt
Showing anger easily
Dislike unnecessary things
Loves making friends but rarely shows it
Daring and stubborn
Ambitious
Realizing dreams and hopes
Sharp
Loves entertainment and leisure
Romantic on the inside not outside
Superstitious and ludicrous
Spendthrift
Learns to show emotions

MARCH
Attractive personality
Affectionate
Shy and reserved Secretive
Naturally honest, generous and sympathetic
Loves peace and serenity
Sensitive to others
Loves to serve others
Not easily angered
Trustworthy
Appreciative and returns kindness
Observant and assesses others
Revengeful
Loves to dream and fantasize
Loves traveling  
Loves attention
Hasty decisions in choosing partners  
Loves home decors
Musically Talented
Loves special things  
Moody


I think he fits march better.



Also he is the year of the wood horse:

The Horse:
Sporty, sociable, uncomfortable around opposite sex, helpful, motivated by advancement, dependable, methodical and quirky.

The Wood Horse 1954 and 2014:
Like other Animal Signs, wood gives this Horse stability and strength. These Horses don't have such a hard time making decisions that other Horses do. Their ability to stick to their guns makes them a bit more predictable than other Horses. They are better equipped to maintain a disciplined approach to their lives and the lives of those they are associated with. They have no problem working with other people and to a point, Wood Horses will stick to their scheduled tasks more readily than other Animal Signs, or even other elemental Horse signs. Because of their ability to easily adapt and work with other people they are successful in their professional and personal lives. Still, don't try to tell a Wood Horse what to do unless you are his superior. Otherwise, he will simply gawk at your attempt to boss him around.