Having a template sensor with an last_update atribute getting the time from {{ now() }}, its displayed like:
2020-04-29 19:03:56.035485+02:00 (without a T).
Looking at other timestamps for other (non template sensors) the time format is with a “T” like:
2020-04-29T04:26:03+02:00
How do I get my template time attribute to use the “T”-format?
Looking at the time section of the template docs, you probably need to use the timestamp_custom filter. But that also means reading up on the strftime docs to build the format you’re expecting.
That is the default display format for a Python datetime object, which is what now() returns. If you would prefer an ISO formatted string representing the value of the datetime object, try:
{{ now().isoformat() }}