The .NET Library EPPlus implements a conversation from the string definition to the built in number. See class ExcelNumberFormat:
internal static int GetFromBuildIdFromFormat(string format)
{
switch (format)
{
case "General":
return 0;
case "0":
return 1;
case "0.00":
return 2;
case "#,##0":
return 3;
case "#,##0.00":
return 4;
case "0%":
return 9;
case "0.00%":
return 10;
case "0.00E+00":
return 11;
case "# ?/?":
return 12;
case "# ??/??":
return 13;
case "mm-dd-yy":
return 14;
case "d-mmm-yy":
return 15;
case "d-mmm":
return 16;
case "mmm-yy":
return 17;
case "h:mm AM/PM":
return 18;
case "h:mm:ss AM/PM":
return 19;
case "h:mm":
return 20;
case "h:mm:ss":
return 21;
case "m/d/yy h:mm":
return 22;
case "#,##0 ;(#,##0)":
return 37;
case "#,##0 ;[Red](#,##0)":
return 38;
case "#,##0.00;(#,##0.00)":
return 39;
case "#,##0.00;[Red](#,#)":
return 40;
case "mm:ss":
return 45;
case "[h]:mm:ss":
return 46;
case "mmss.0":
return 47;
case "##0.0":
return 48;
case "@":
return 49;
default:
return int.MinValue;
}
}
When you use one of these formats, Excel will automatically identify them as a standard format.