diff --git a/apps/sim/lib/file-parsers/sheet-display-text.test.ts b/apps/sim/lib/file-parsers/sheet-display-text.test.ts
index c44b887dc0e..355da1cd223 100644
--- a/apps/sim/lib/file-parsers/sheet-display-text.test.ts
+++ b/apps/sim/lib/file-parsers/sheet-display-text.test.ts
@@ -230,4 +230,19 @@ describe('normalizeSheetDisplayText', () => {
     expect(inside.w).toBe('2026-03-04')
     expect(outside.w).toBe('3/5/2026')
   })
+
+  it('keeps the file text for a date format longer than Excel allows', () => {
+    const sheet = XLSX.utils.aoa_to_sheet([['a']])
+    const at = new Date(Date.UTC(2024, 2, 15, 14, 30))
+    sheet.A1 = { t: 'd', v: at, z: `m/d/yyyy h:mm${'"x"'.repeat(100)}`, w: '3/15/2024 14:30' }
+    sheet.B1 = { t: 'd', v: at, z: `m/d/yyyy h:mm${'"x"'.repeat(80)}`, w: '3/15/2024 14:30' }
+    sheet.C1 = { t: 'd', v: at, z: `[h]:mm${'"x"'.repeat(100)}`, w: '14:30' }
+    sheet['!ref'] = 'A1:C1'
+
+    normalizeSheetDisplayText(sheet, XLSX.utils.decode_range('A1:C1'), XLSX.utils)
+
+    expect(sheet.A1.w).toBe('3/15/2024 14:30')
+    expect(sheet.B1.w).toBe('2024-03-15T14:30:00')
+    expect(sheet.C1.w).toBe('14:30')
+  })
 })
diff --git a/apps/sim/lib/file-parsers/sheet-display-text.ts b/apps/sim/lib/file-parsers/sheet-display-text.ts
index e9a16134381..7c968e4a1a6 100644
--- a/apps/sim/lib/file-parsers/sheet-display-text.ts
+++ b/apps/sim/lib/file-parsers/sheet-display-text.ts
@@ -23,6 +23,15 @@ const GENERAL_SIGNIFICANT_DIGITS = 15
 
 const ELAPSED_TOKEN = /\[(h+|m+|s+)\]/i
 
+/**
+ * Excel rejects a custom number format longer than 255 characters, so a longer
+ * `cell.z` is not a real format. It comes verbatim from the workbook's
+ * `styles.xml`, and parsing it would let one cell cost unbounded CPU.
+ */
+const MAX_NUMBER_FORMAT_LENGTH = 255
+
+const TIME_SEPARATOR = /[:\s]/
+
 /**
  * Strips the parts of a number format that carry no date tokens: quoted
  * literals, backslash escapes, bracketed colour/condition/elapsed sections and
@@ -48,14 +57,25 @@ export function isTimeOnlyFormat(format: string): boolean {
   if (/[yd]/.test(tokens)) return false
   if (!/[hms]/.test(tokens)) return false
   for (const match of tokens.matchAll(/m+/g)) {
-    const before = tokens.slice(0, match.index).replace(/[:\s]+$/, '')
-    const after = tokens.slice(match.index + match[0].length).replace(/^[:\s]+/, '')
-    const isMinutes = before.endsWith('h') || after.startsWith('s')
-    if (!isMinutes) return false
+    if (!isMinutesRun(tokens, match.index, match.index + match[0].length)) return false
   }
   return true
 }
 
+/**
+ * Whether the `m` run at `[start, end)` sits next to an hour or a seconds
+ * token, skipping `:` and whitespace. Each separator span is walked at most
+ * once from each neighbouring run, so classifying a whole format is linear.
+ */
+function isMinutesRun(tokens: string, start: number, end: number): boolean {
+  let before = start - 1
+  while (before >= 0 && TIME_SEPARATOR.test(tokens[before])) before--
+  if (tokens[before] === 'h') return true
+  let after = end
+  while (after < tokens.length && TIME_SEPARATOR.test(tokens[after])) after++
+  return tokens[after] === 's'
+}
+
 const MS_PER_SECOND = 1000
 
 /**
@@ -106,6 +126,9 @@ function isGeneralFormat(format: unknown): boolean {
  * Elapsed-time formats (`[h]:mm`, `[mm]:ss`) are durations, not moments;
  * `cellDates` still parses them into a `Date`, so their `w` (`30:00`) is kept.
  *
+ * A date whose format is longer than Excel allows is not a real format, so it
+ * keeps the text the file rendered for it too.
+ *
  * A number with no format at all is treated as General too. Every other
  * number keeps the text the file rendered for it, so a LibreOffice workbook
  * indexes as LibreOffice showed it (`0,5` in a German locale).
@@ -133,7 +156,12 @@ export function normalizeSheetDisplayText(
 
       if (cell.t === 'd' && cell.v instanceof Date) {
         const format = typeof cell.z === 'string' ? cell.z : undefined
-        if (format !== undefined && ELAPSED_TOKEN.test(format)) continue
+        if (
+          format !== undefined &&
+          (format.length > MAX_NUMBER_FORMAT_LENGTH || ELAPSED_TOKEN.test(format))
+        ) {
+          continue
+        }
         cell.w = isoDateText(cell.v, format)
       } else if (cell.t === 'n' && typeof cell.v === 'number' && isGeneralFormat(cell.z)) {
         cell.w = generalNumberText(cell.v)