Two kinds of Bengali date are in use in Bangladesh today — one from the official revised calendar, the other from the Sanatani panjika. The official date is a fixed solar calendar, in which the 1966 reform set the first five months to 31 days and the rest to 30 days, and Pohela Boishakh is fixed to 14 April every year.
The Sanatani panjika, on the other hand, depends on actual astronomical calculation. Here a month changes at the moment the sun moves from one zodiac sign to the next (the sankranti), not by a fixed number of days. As a result each month's length varies slightly and the date stays aligned with the true position of the sky.
Because the basis of these two systems differs, the official and Sanatani dates usually differ by one to two days. The Sanatani panjika is followed for religious vratas, pujas and tithi, while the official date is used for everyday and administrative work. MandirBD shows both side by side so there is no confusion.