In Journey Builder you can add a decision split to check for today’s date
SIGNUP_DATE equal Today
and use the Update Contact activity to set a field value with “current date time”.
However, the time SFMC is looking for when evaluating “Today”, and times set in Journey Builder are in CST. So if your date field is local time (e.g. AEST), but CST time means the current date is yesterday, this could affect how customers flow through your journey.