The situation is as follows:

does not draw:

T_to = timestamp(year + 1, 3, 1, 0, 0)

does not draw:

T_to = timestamp(year, 12, 31, 0, 0) // ? current 2024 December = 31 с. day

draws:

T_to   = timestamp(year, 12, 30, 0, 0)

code :

//@version=5
indicator("Q time high/low fix", overlay=true )

// Bar counter in period
var int count_bar = na

// for high and low
var float high_in_period = na
var float low_in_period = na

// Quarter period: from December 1 of the current year to March 31 of the next year
T_from = timestamp(year, 12, 1, 0, 0)    //  Time from [t...]
T_to   = timestamp(year +1, 3, 1, 0, 0)  //  Time to   [...t]. Correct end of March next year

// Debug marks for checking values
if (time == T_to)    //  ???
    label.new(bar_index, high, "Time: " + str.tostring(time) + "nDate: " + str.format("{0}-{1}-{2}", year(time), month(time), dayofmonth(time)), color=color.yellow, style=label.style_label_down)


// If the current bar falls within the period
if (time >= T_from and time <= T_to)
    
    // Initializing values ​​on the first bar of the period
    if na(count_bar)
        count_bar := 0
        high_in_period := high
        low_in_period := low
    
    // Increasing the bar counter
    count_bar += 1
    
    // Updating high and low
    high_in_period := math.max(high_in_period, high)
    low_in_period  := math.min(low_in_period, low)

// After the period ends, we draw lines and reset the counters
if (time > T_to and not na(count_bar))

    // lines high and low
    line.new(x1=T_from, y1=high_in_period, x2=T_to, y2=high_in_period, color=color.green, width=2, xloc=xloc.bar_time)
    line.new(x1=T_from, y1=low_in_period,  x2=T_to, y2=low_in_period, color=color.red, width=2, xloc=xloc.bar_time)

    // Reset the counters
    count_bar := na
    high_in_period := na
    low_in_period := na

//    END

I noticed that bars from January 1 to the required date are not taken into account for analysis in the code.

How to draw high and low lines in a given time range
with the transition to the new year (year+1) ?