You are passing a invalid date in text format. Check the date string properly... Use DateTime.Parse instead of Convert.ToDateTime() in these lines of code
DateTime joiningDate = Convert.ToDateTime(joiningdate);
DateTime firstpromotionDate = Convert.ToDateTime(firstpromotiondate);
The DateTime.Parse(String) method tries to convert the string representation of a date and time value to its DateTime equivalent. The string to be parsed can take any of the following forms:
A string with a date and a time component.
A string with a date but no time component.
A string with a time but no date component.
A string that includes time zone information and conforms to ISO 8601. For example, the first of the following two strings designates the Coordinated Universal Time (UTC); the second designates the time in a time zone seven hours earlier than UTC:
A string that includes the GMT designator and conforms to the RFC 1123 time format. For example:
Sat, 01 Nov 2008 19:35:00 GMT
A string that includes the date and time along with time zone offset information. For example:
03/01/2009 05:42:00 -5:00
This method attempts to parse s completely and avoid throwing a FormatException. It ignores unrecognized data if possible and fills in missing month, day, and year information with the current date. If s contains only a date and no time, this method assumes 12:00 midnight. If s contains only a time and no date, this method assumes the current date. If s includes a date component with a two-digit year, it is converted to a year in the current culture's current calendar based on the value of the Calendar.TwoDigitYearMax property. Any leading, inner, or trailing white space character in s is ignored. The date and time can be bracketed with a pair of leading and trailing NUMBER SIGN characters ('#', U+0023), and can be trailed with one or more NULL characters (U+0000).