Part 2

"THEY ASK YOU ABOUT THE NEW MOONS. SAY: THEY ARE INDICATIVE OF TIME FOR THE PEOPLE, AND OF THE HAJJ. AND IT IS NOT RIGHTEOUSNESS THAT YOU COME INTO YOUR HOUSES FROM THEIR BACK SIDE BUT RIGHTEOUSNESS IS THAT ONE FEARS ALLAH. AND COME TO THE HOUSES THROUGH THEIR DOORS. AND FEAR ALLAH SO THAT YOU MAY BE SUCCESSFUL. (SURAH AL-BAQARAH:189)

Continued from before...

While in matters of religious matters, the Ibadat, the lunar calendar has been declared obligatory, it has also been favoured in social and business transactions because to serves as a basis for the acts of Islamic worship, as well as a symbol of Islamic identity. The use of solar calendar is not prohibited, the only condition being that its use should not lead to complete cessation of use of lunar calendar, as this would lead to disruption in obligatory Ibadat like Hajj and fasting. If Muslims use solar calendar in office settings, particularly when they have to do transactions with non-Muslims, but use the lunar calendar for the rest of their office correspondence and private dealings not only will it lead to earning the Thawab (reward) of performing a Fard ala al Kifayah (an obligation which if performed by some Muslims, suffices for others), but will also lead to preservation of their national identity.

From Ma'ariful Quran, by Mufti Muhammad Shafi RE