▁usually ▁in ▁order ▁to ▁increase ▁their ▁value , ▁in ▁which ▁the ▁post mark ▁has ▁been ▁alter ed ▁in ▁some ▁way ; ▁for ▁example , ▁by ▁changing ▁the ▁date . ▁ ▁Pract ical ▁uses ▁ ▁The ▁post mark ▁is ▁often ▁considered ▁as ▁an ▁official ▁confirm ation ▁that ▁a ▁cover ▁( letter , ▁packet , ▁etc .) ▁ma iled ▁item ▁was ▁ma iled ▁at ▁a ▁given ▁location ▁at ▁a ▁specific ▁date . ▁For ▁example , ▁the ▁date ▁of ▁the ▁post mark ▁can ▁be ▁quite ▁important . ▁In ▁the ▁United ▁States , ▁the ▁In ternal ▁Re venue ▁Service ▁will ▁still ▁consider ▁income ▁tax ▁returns ▁as ▁file d ▁on ▁time ▁though ▁it ▁receives ▁them ▁late ▁if ▁they ▁are ▁post mark ed ▁on ▁time , ▁and ▁this ▁date ▁( with , ▁perhaps , ▁other ▁proofs ▁of ▁ma iling ), ▁may ▁have ▁significance ▁as ▁regards ▁legal ▁fil ings ▁and ▁proofs ▁of ▁service ▁( though ▁in ▁this ▁case ▁the ▁date ▁may ▁viewed ▁as ▁" on ▁time " ▁if ▁the ▁date ▁of ▁the ▁post mark ▁is ▁no ▁more ▁than ▁one ▁day ▁after ▁the ▁date ▁service ▁is ▁supposed ▁to ▁have ▁been ▁made ). ▁Ent ries ▁into ▁swe ep st akes ▁and ▁cont ests , ▁and ▁jur ied ▁art ▁exhib itions , ▁may ▁like wise ▁have ▁a ▁" post mark ▁dead line ," ▁and ▁in ▁at ▁least ▁one ▁case ▁it ▁might ▁have ▁significance ▁regarding ▁the ▁date ▁of ▁class ▁withdraw al . ▁ ▁Similar ▁marks ▁▁ ▁A ▁post mark ▁shouldn ' t ▁be ▁confused ▁with ▁the ▁k iller ▁which ▁are ▁lines , ▁bars ,