Most models run on a timeline: one column per month, quarter or year, with a real date at the top. Type the first period-end date and build the rest with =EOMONTH(B4,3), which jumps to the last day of the month three months later. Adding 91 days instead drifts off month-end within a year.
A flag is a row of 1s and 0s that says whether something is happening in that period: construction or operations, a December quarter, a loan being repaid. =IF(B4>=$B$13,1,0) turns on from a start date. Multiply a row by its flag, =B5*$B$14, and the row is zero until the flag switches on. No IFs scattered through the model.
A switch is a single input cell, 1 or 0, that turns an assumption on or off for the whole model: (1+$B$15*$B$16) adds a price increase only when B16 is 1.
A running count, =SUM($B$5:B5), anchors the start and lets the end move, so each column counts the periods so far.