Is this a nanny share? Or are you the only employer?

Pay her from the time she walks in.

Just tell her that being on time is paramount and don't be nice about it. I personally am never late and it bugs me when other people are. Lucky for me, DD's nanny feels the same way and has never once been late in over three years.

We eventually fired our nanny for this and I was so relieved. I was so stressed out all the time from always being late for work. And it wasn't a 'flexibility works both ways' situation - she was supposed to work until 5"45 but if I walked in at 5"30 she was sitting there with her car keys in hand.

Are you ever late arriving home. Flexibility works both ways.

Not excusing her being late by any means, but is she late because she perceives you dont need her until you actually leave the house? So if you asked her to start at 8 but you actually leave at 8.30 and she is there before 8.30? Or is she late where you are then late getting to work?
I am asking because if you need to leave at the time she arrives I would tell her she needs to be there 20 mins earlier from now on. If she is continually late you may need to find someone else.
If she is late because she feels you are handling the mornings and she is kind of twiddling her thubbs until you leave then you need to have a talk about what she needs to get done once she walks in the door.

Tell her she needs to get to work at 8am if you need her to show at 8:30, AND tell her you'll pay her for the time she's actually there.

Or, fire her.

Stop paying her for that time, first of all.

20 minutes some days. But otherwise great with the kids. How do you remedy this when constantly reminding her to be on time doesn't seem to work?