"Standard or daylight time?" UTC, of course...
Seriously, you have to replace all of your toobs at the same time or else you will break the magic spell.
Even more seriously, it does depend - on the amp and the level of granularity in setting bias, etc. but if you're even asking this question then you are not totally sick/obsessive (yet) and therefore no, you don't absolutely have to replace all tubes at the same time. You would ideally have all the tubes degrade on a coordinated schedule but since they typically do not the best thing is to look for "savings opportunities" and to acquire sufficient stock to handle these as they happen. This is not something to obsess over - you can keep track of "tube hours" but in the end do you care more than anything else about that or do just enjoy that sound and want to hear more?
Seriously, you have to replace all of your toobs at the same time or else you will break the magic spell.
Even more seriously, it does depend - on the amp and the level of granularity in setting bias, etc. but if you're even asking this question then you are not totally sick/obsessive (yet) and therefore no, you don't absolutely have to replace all tubes at the same time. You would ideally have all the tubes degrade on a coordinated schedule but since they typically do not the best thing is to look for "savings opportunities" and to acquire sufficient stock to handle these as they happen. This is not something to obsess over - you can keep track of "tube hours" but in the end do you care more than anything else about that or do just enjoy that sound and want to hear more?