Recurring tasks (#847): Fix php error (substract is not always defined)
Recurring tasks (#847): Fix php error (substract is not always defined)

dromek/kanboard,cranca/kanboard,eSkiSo/kanboard,wiebkem/kanboard,mbrzuchalski/kanboard,skobeo/kanboard,EpocDotFr/kanboard,mcfog/kanboard,mcfog/kanboard,julesverhaeren/kanboard,mcorteel/kanboard,goofy-bz/kanboard,erwang/kanboard,leangjia/kanboard,jtourt/kanboard,ChMat/kanboard,Shaxine/kanboard,wepika/kanboard,thylo/kanboard,cHolzberger/kanboard,renothing/kanboard,Shaxine/kanboard,dromek/kanboard,fguillot/kanboard,corretgecom/kanboard,jmptrader/kanboard,cdgeelen/kanboard,kasunf/KanboardBOI,Busfreak/kanboard,onedal88/kanboard,stinnux/kanboard,cogumm/kanboard,jtourt/kanboard,djpadz/kanboard,patoi/kanboard,wepika/kanboard,goofy-bz/kanboard,arcanneero/kanboard,vjscjp09/kanboard-1,Nerdingher/kanboard,kanboard/kanboard,jclafuente/kanboard,markmarco16/kanboard,jclafuente/kanboard,hussam789/kanboard,xavividal/kanboard,stinnux/kanboard,renothing/kanboard,thylo/kanboard,eSkiSo/kanboard,filhocf/kanboard,marienfressinaud/kanboard,vaikan/kanboard,kanboard/kanboard,filhocf/kanboard,djpadz/kanboard,lianguan/kanboard,stinnux/kanboard,julesverhaeren/kanboard,libin/kanboard,lianguan/kanboard,wtmmac/kanboard,corretgecom/kanboard,kanbantestnew/kanban,cHolzberger/kanboard,jclafuente/kanboard,eSkiSo/kanboard,jmptrader/kanboard,wepika/kanboard,Shaxine/kanboard,xavividal/kanboard,djpadz/kanboard,filhocf/kanboard,julesverhaeren/kanboard,eSkiSo/kanboard,crwilliams/kanboard,libin/kanboard,xavividal/kanboard,ChMat/kanboard,jmptrader/kanboard,oliviermaridat/kanboard,jvizueta/kanboard,hussam789/kanboard,kclabrador/kan-test,cranca/kanboard,jtourt/kanboard,JeroenKnoops/kanboard,sanbiv/kanboard,arcanneero/kanboard,Busfreak/kanboard,marienfressinaud/kanboard,Busfreak/kanboard,onedal88/kanboard,vjscjp09/kanboard-1,vjscjp09/kanboard-1,patoi/kanboard,cdgeelen/kanboard,kclabrador/kan-test,mcorteel/kanboard,leangjia/kanboard,onedal88/kanboard,wtmmac/kanboard,fguillot/kanboard,markmarco16/kanboard,vjscjp/kanboard,sanbiv/kanboard,onedal88/kanboard,crwilliams/kanboard,thylo/kanboard,BlueTeck/kanboard,jvizueta/kanboard,EpocDotFr/kanboard,cHolzberger/kanboard,renothing/kanboard,cdgeelen/kanboard,mbrzuchalski/kanboard,patoi/kanboard,kanboard/kanboard,JeroenKnoops/kanboard,vaikan/kanboard,alu0100502114/kanboard,dromek/kanboard,skobeo/kanboard,alu0100502114/kanboard,mcorteel/kanboard,kasunf/KanboardBOI,oliviermaridat/kanboard,creador30/kanboard,BlueTeck/kanboard,vaikan/kanboard,SamuelMoraesF/kanboard,cHolzberger/kanboard,oliviermaridat/kanboard,Nerdingher/kanboard,wiebkem/kanboard,crwilliams/kanboard,fguillot/kanboard,SamuelMoraesF/kanboard,cogumm/kanboard,Nerdingher/kanboard,kasunf/KanboardBOI,vjscjp/kanboard,kanbantestnew/kanban,crwilliams/kanboard,fabiano-pereira/kanboard,BlueTeck/kanboard,creador30/kanboard,manishlad/kanboard,wepika/kanboard,lianguan/kanboard,fabiano-pereira/kanboard,jmptrader/kanboard,manishlad/kanboard,erwang/kanboard