Masik Shivratri is the Krishna Chaturdashi of every lunar month - the monthly observance of which Maha Shivaratri is the annual one. Because the worship belongs to Nishita Kaal, the muhurta at the exact middle of the night, the day it falls on is the day whose NIGHT the Chaturdashi fills, which is often not the day the Chaturdashi happens to be running at sunrise. This calendar lists the upcoming dates for your city, each with its computed Pradosh Kaal and Nishita Kaal windows and the rule that chose that night over its neighbour.
Most fasts are dated by the tithi running at sunrise, and that is the correct rule for an observance kept through the day. Shivaratri is not one of those. Kane, History of Dharmasastra V.i p.73, states the other half of the rule directly: all vratas to be performed in the evening or night have to be performed on the tithi existing in the evening or night, even though it may be mixed up with another tithi.
A tithi is roughly twenty-four hours long but never exactly a day, so a Krishna Chaturdashi routinely touches two consecutive nights. Which of the two is the vrat night is a real question with a real answer, and it is the answer this calendar computes rather than assuming the sunrise day.
Kane reports the test at V.i pp.229-230, from Madhava's Kalanirnaya and cross-referenced to the Nirnaya Sindhu. It is applied in strict order, and the first rung that separates the two candidate nights wins:
Nishita is not a single agreed window. Kane records that it has several classical definitions and does not pick between them. The modern almanac convention is the eighth of the night's fifteen muhurtas, a window of two ghatis centred exactly on the midpoint between sunset and the next sunrise. The alternative Kane quotes is a span of eight ghatikas beginning half a prahara into the night, which sits noticeably earlier.
Every row on this page prints both windows rather than choosing one. The ladder above is run on the eighth-muhurta reading, and that is stated on the row, so a reader who follows the other definition can see exactly what would change.
Maha Shivaratri is one particular Masik Shivratri: the Krishna Chaturdashi of Magha by the amanta reckoning, which is the same night as Phalguna Krishna Chaturdashi by the purnimanta reckoning. The twelve monthly Shivaratris and the great one are dated by the same ladder; only the month differs.
The night vigil is traditionally kept in four praharas, the four equal quarters of the night from sunset to sunrise, with worship offered in each. Those quarters follow the actual length of the night at your location, which is why this calendar asks for a city rather than printing a fixed clock time.
Masik Shivratri is the Shivaratri kept every lunar month, on the Krishna Chaturdashi - the fourteenth tithi of the waning fortnight. Worship is offered at Nishita Kaal, the middle of the night. Maha Shivaratri is the same observance in the month of Magha (amanta) or Phalguna (purnimanta).
Because the two are answering different questions. A calendar that prints the tithi running at sunrise will give a different day from one that asks which night the Chaturdashi actually fills, and Shivaratri is a night observance. This page runs Kane's four-rung ladder from Madhava's Kalanirnaya (History of Dharmasastra V.i pp.229-230) and names, on every row, which rung decided it - so you can see exactly where a disagreement comes from.
Nishita Kaal is the eighth of the night's fifteen muhurtas, a window of about forty-eight minutes centred on the exact midpoint between sunset and the next sunrise. It is not a fixed clock time - it shifts with your city's sunset and lengthens in winter. This page also prints the alternative eight-ghatika reading Kane records, because he does not choose between the definitions.
No. Only a city, so that sunset, sunrise and the muhurta windows derived from them are computed for your location.
For educational purposes. Astrological interpretations are traditional and not a substitute for professional advice.