Is it correct to omit preposition before date?

Today, I received a mail from Steam and noticed that there was no preposition before the date. This is the part of the mail:

SPECIAL PROMOTION! Offer ends 26 Apr 2:00am KST

Shouldn't it be:

SPECIAL PROMOTION! Offer ends on 26 Apr at 2:00am KST

If it's okay to omit the prepositions before a date, are these sentences grammatically correct:

The gift will arrive 26 Apr 2:00am KST.
He sent the message 26 Apr 2:00am KST.
It happens every Monday 2:00am KST.

The 'on' before the date (and the 'at' before the time) was dropped for effect -- this is a 'telegraphic style' that has become accepted in modern English in certain contexts through repeated use in the mass media. However it is not recommended for formal use in settings where absolutely correct grammar is called for.