White Day Fasting: Lunar or Solar Calendar?
Question

I am a "New" Muslim (or recently converted) and I thank Allah for guiding me to this truly happiest move I've made in my life. Among all the other ways to worship the Allah, I appreciate very much the issue of fasting. I have truly enjoyed my first Ramadan this year and went through all 30 days with pride and great faith. I'm also very interested in voluntary fasting and besides fasting Thursdays, I've been also fasting during the White Days (13,14,15 every month). However, I've realized that I've been fasting 13,14,15 according to Georgian, normal calendar, and not according to an Islamic one! Is what I have done really wrong? Does it annul my fasting? What shall I do in the future? Thank you very much in advance!
Praise be to Allah.First of all, congratulations on the blessing of Allah which you have received, as He has guided you to this religion and caused you to enjoy the sweetness of obedience and worship through fasting, which is one of the greatest forms of worship. Here we will explain in brief the virtues of fasting.
Virtues of fasting
The virtues of fasting are immense, and a number of sound hadiths state this, such as the following:

Fasting is something which is just for Allah, and Allah will give multiple rewards without measure for it. The hadith says: [Allah says:] Fasting is Mine and it is I Who give reward for it. (Al-Bukhari, Fath, no. 1904)

There is nothing equivalent to fasting. (Al-Nasai, 4/165; see also Sahih al-Targhib, 1/413)

The supplication (du’a) of the fasting person will not be refused. (Reported by al-Bayhaqi, 3/345; see also al-Silsilah al-Sahihah, 1797)
The one who fasts has two joys: the joy of breaking his fast and, when he meets his Lord, the joy of having fasted. (Reported by Muslim, 2/807)
Fasting will be an intercession for the slave on the Day of Resurrection, when it will say: O Lord, I stopped him from eating and drinking during the day, so let me intercede for him. (Reported by Ahmad, 2/174; al-Bayhaqi classed its isnad as hasan in al-Mujtama, 3/181; see also Sahih al-Targhib, 1/411)
The smell (coming from the mouth of) the fasting person is better in the sight of Allah than the scent of musk. (Muslim, 2/808).
Fasting is a protection and a strong fortress against the Fire. (Reported by Ahmad, 2/402; see also Sahih al-Targhib, 1/411 and Sahih al-Jami’, 3880)
Whoever fasts one day for the sake of Allah, Allah will keep his face seventy years distance away from the Fire because of it. (Reported by Muslim, 2/808)
Whoever fasts a day seeking the pleasure of Allah and then dies [i.e. that is the last thing he does] will enter Paradise. (Reported by Ahmad, 5/391; see also Sahih al-Targhib, 1/412)
In Paradise, there is a gate called al-Rayyan, through which those who fast will enter, and no-one else will enter through it. After they have entered, it will be locked and no-one else will enter it. (Al-Bukhari, Fath, no. 1797).
Voluntary fasts will make up for any shortfall in obligatory fasts. Examples of voluntary fasts include ‘Ashura , the Day of ‘Arafah , al-Ayam al-Bid (the White Days), Mondays and Thursdays , six days of Shawwal , and increased fasting during the months of al-Muharram and Sha’ban.

What are the White Days?
What is meant by al-Ayyam al-Bid is the 13th, 14th, and 15th of each lunar month, because Allah says (interpretation of the meaning): “They ask you about the new moons. Say: These are signs to mark fixed periods of time for mankind and for the pilgrimage.” [al-Baqarah 2:189] 
So the timing of certain acts of worship and the ‘iddah (waiting period for women who have been divorced or widowed, etc.) are worked out according to the lunar months of the Hijri or Islamic calendar, not the solar months of the Gregorian or Western calendar. 
The fasts which you have observed according to the solar calendar most likely do not correspond with the lunar calendar, but in any case you will be rewarded, in sha Allah, for the days that you have fasted, because you have fasted them voluntarily for the sake of Allah. But if you wish to earn the reward that is specifically for fasting Ayyam al-Bid, which the Prophet (peace and blessings of Allah be upon him) urged us to do, you should find out the timings according to the lunar calendar and fast according to it.
We ask Allah to increase His blessings upon you, to help you adhere to His religion, to give you the strength to do that which will please Him and to increase your reward, for He is the All-Hearing Who answers prayers. Do not forget us in your prayers whilst you are fasting. May Allah bless our Prophet Muhammad.