Quick answer

No: the standard forms entitle the contractor to finish on or before the completion date, require the certifier to certify completion once it occurs, and provide for monthly certification and payment of work properly done regardless of progress, so there is no facility to reduce or delay certification to suit the employer's cashflow

No. The standard forms entitle the contractor to complete on or before the completion date, so it may finish early: JCT 2011 clause 2.4, the ICE 6th and 7th Editions clause 43, the Engineering and Construction Contract clause 30.1 and GC/Works/1 condition 34(1) all point that way. Once the contractor has finished, the certifier must certify completion, under the ICE clause 48, JCT 2011 clause 2.30, GC/Works/1 condition 39 and the Engineering and Construction Contract clause 30.2, with no option to withhold. Payment is not tied to the employer's cashflow either: the forms provide for certification and payment of work properly executed, usually monthly, regardless of whether the contractor is ahead of programme, under JCT 2011 clause 4.9.2, the ICE clause 48, GC/Works/1 condition 47 and the Engineering and Construction Contract clause 50. An employer that wants to manage cashflow can use milestone or stage payments and a cashflow forecast, as FIDIC 4th Edition clause 14.3 provides, but there is no facility for the certifier to reduce the amount certified or delay certification to suit the employer's ability to pay.