I have solved this error by my own. I have removed Inbuilt function like today and now with my own value from code side.
Microsoft.SharePoint.Utilities.SPUtility.CreateISO8601DateTimeFromSystemDateTime(
DateTime.Now.AddMonths(-1))
More Complete Sample
string queryString_getPersonalAlerts = string.Format(@"<Where><Or>
<And><Eq><FieldRef Name='{0}' />
<Value Type='Choice'>Daily</Value></Eq><And><Geq><FieldRef Name='{1}' />
<Value Type='DateTime'>{2}</Value></Geq><And><Geq><FieldRef Name='{1}' />
<Value IncludeTimeValue='TRUE' Type='DateTime'>{2}</Value></Geq><Leq><FieldRef Name='{1}' />
<Value IncludeTimeValue='TRUE' Type='DateTime'>{3}</Value></Leq></And></And></And><Or><And><Eq><FieldRef Name='{0}' />
<Value Type='Choice'>Weekly</Value></Eq><And><Eq><FieldRef Name='{1}' />
<Value Type='DateTime'>{4}</Value></Eq><And><Geq><FieldRef Name='{1}' />
<Value IncludeTimeValue='TRUE' Type='DateTime'>{4}</Value></Geq><Leq><FieldRef Name='{1}' />
<Value IncludeTimeValue='TRUE' Type='DateTime'>{5}</Value></Leq></And></And></And><And><Eq><FieldRef Name='{0}' />
<Value Type='Choice'>Monthly</Value></Eq><And><Eq><FieldRef Name='{1}' />
<Value Type='DateTime'>{6}</Value></Eq><And><Geq><FieldRef Name='{1}' />
<Value IncludeTimeValue='TRUE' Type='DateTime'>{6}</Value></Geq><Leq><FieldRef Name='{1}' />
<Value IncludeTimeValue='TRUE' Type='DateTime'>{7}</Value></Leq></And></And></And></Or></Or></Where>",
PersonalAlerts.Fields.DeliveryFrequency.ToString(),
PersonalAlerts.Fields.LastDeliveryTime.ToString(),
SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now.AddDays(-1)),
SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now.AddDays(-1).AddMinutes(60)),
SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now.AddDays(-7)),
SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now.AddDays(-7).AddMinutes(60)),
SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now.AddMonths(-1)),
SPUtility.CreateISO8601DateTimeFromSystemDateTime(DateTime.Now.AddMonths(-1).AddMinutes(60)));