Date(Date#(Calls.Date, 'DD/MM/YYYY'), 'YYYYMM') as Calls.Date, SKILL_NUMBER, ApplyMap('MapCallSkillNoToName', SKILL_NUMBER) as SKILL_NAME, Calls.isNonClaim, ASA, sum(CALLS_RECEIVED) as CALLS_RECEIVED, sum(CALLS_ANSWERED) as CALLS_ANSWERED, sum(CALLS_ABANDONED) as CALLS_ABANDONED, sum(CALLS_TRANSFERRED) as CALLS_TRANSFERRED,
sum(ASA) as SumASA, max(MAXIMUM_ANSWER_DELAY) as MAXIMUM_ANSWER_DELAY, sum(CUST_ABANDONED) as CUST_ABANDONED, sum(CALLS_ANSWERED_WITHIN_SLA) as CALLS_ANSWERED_WITHIN_SLA, sum(CUST_OFFERED) as CUST_OFFERED
FROM [..\QVDs\T_Call.qvd] (qvd) where Date(Date#(Calls.Date, 'DD/MM/YYYY'), 'DD/MM/YYYY') >= '28/12/2013'
An statement like yours - '28/12/2013' - worked only if this is the default date-format within the qvw. If not it won't be interpretet as date it would be anything else and often be only a string. This is main reason why you shouldn't never use date-formats within any matchings or calculation else pure numeric values which you could get with:
num(date) or num(date#(date, Format))
This avoids many problems with this format-stuff especially if you have multiple date-fields and it will be mostly perform better because there is no parsing-overhead.
In the long-term you shoud consider to change your date-logics but for now should the suggestion from bwisenoSimenKG work, too.