▁people ▁can ▁print ▁from ▁their ▁own ▁PC s ▁( called ▁PC ▁Post age ▁in ▁the ▁United ▁States , ▁these ▁services ▁were ▁offered ▁by ▁such ▁companies ▁as ▁St amps . com ▁and ▁Ne op ost , ▁Inc .). ▁These ▁indic ia ▁are ▁not ▁always ▁post mark ed ▁by ▁the ▁Post ▁Office ▁but ▁if ▁put ▁into ▁the ▁mail stream ▁later ▁than ▁the ▁date ▁listed ▁on ▁them , ▁they ▁are ▁post mark ed ▁about ▁ 5 0 % ▁of ▁the ▁time . ▁ ▁Because ▁of ▁this , ▁it ▁is ▁a ▁bad ▁idea ▁to ▁try ▁and ▁use ▁the ▁date ▁on ▁your ▁post age ▁as ▁a ▁post mark . ▁ ▁An ▁official ▁example ▁rel ating ▁a ▁num ism atic ▁item ▁to ▁post marks ▁occurred ▁on ▁April ▁ 1 3 , ▁ 1 9 7 6 ▁when ▁the ▁U . S . ▁issued ▁a ▁new ▁two - d ollar ▁bill . ▁ ▁People ▁could ▁buy ▁the ▁b ills ▁at ▁face ▁value , ▁add ▁a ▁first ▁class ▁stamp ▁( at ▁the ▁time ▁ 1 3 ▁c ents ), ▁and ▁have ▁the ▁combination ▁post mark ed ▁to ▁show ▁they ▁were ▁the ▁first ▁day ▁of ▁issue . ▁ ▁Lar ge ▁numbers ▁of ▁these ▁were ▁produced ▁and ▁they ▁remain ▁common . ▁ ▁In k ▁colour ▁ ▁When ▁the ▁first ▁universal ▁postal ▁system ▁was ▁started ▁in ▁the ▁United ▁Kingdom ▁with ▁its ▁Pen ny ▁Black , ▁the ▁post mark ▁used ▁red ▁in k ▁for ▁contrast . ▁ ▁This ▁was ▁not ▁successful , ▁and ▁the ▁stamp ▁was ▁changed ▁to ▁non - black ▁colours ▁so ▁that ▁the