Relative time/time delta

[maufl]

Is your feature request related to a problem? Please describe.
I would like to be able to send measurements for multiple points in time in the past but I don't have absolute timestamps available.

Describe the solution you'd like
I would like to be able to sent a timedelta (sint) of seconds relative to now.

Additional context
I want to build sensors using ESP32 that send measurements using BLE so that I can avoid WiFi (because of power consumption). Ideally I could do multiple measurements from the low power core with a higher frequency, and then only wake up the high power core with a low frequency to transmit multiple measurements at once.